In chemistry, rates of reaction are proportional, among other things, to temperature; hotter the environment, usually the faster the reaction; hence in colour processing the 'adjustments' for different bath temps; then it is proportional to solution concentrations; which s why in B&W you often have different dev times for mixing solutions at different strengths, say 3:1, 5:1 & 9:1.
But that is at a 'bulk' scale.
In the camera, making an actual exposure, you are trying to 'cheat' the chemistry; where light falls it catalyses the oxidisation reaction of the silver halide emulsion. Where light falls, it starts to oxidise, where it doesn't it doesn't... in the tank, you are doing the same, 'boosting' that reaction, and getting tickled areas to turn black, and untickled ones to stay white... you are actually deliberately trying to cause what in chemistry would be known as an 'incomplete reaction'
For example, chemical equation for burning methane, in oxygen, is CH4+O2=> CO2+H2O.. only what you have to start, you have to have at the end, so while that equation describes the chemicals you start with, and what they get turned in to, it's not quite right as far as the quantities, because I started with CH4, which is one carbon and four hydrogen, at the end, I have one carbon in the CO2, but have lost a few hydrogen with only one in the H2O... It's been two many years since I had to balance equations, and I haven't had enough coffee yet to do the maths,,, but you could, 'balance' the equation to put the correct proportions in so you have as much C,H&O on one side as other.
In photography, however we are trying to 'cheat' the chemistry and deliberately NOT get a 'complete' reaction.. we don't want to turn ALL the silver halide we have to silver oxide.. what we want to do is, turn the sliver halide that has been partially oxidised by light into silver oxide, and NOT oxidize that which hasn't..... Good game, good game......... we are trying, because we are actually relying on it to create a 'partial reaction'.. So we start with an excess of solution.. there's enough oxidsing agent it, to turn every bit of silver halide into silver oxide... but we don't want t ALL black, so we have to let the solution get to work, but stop it before it's finished... and hopefuly we have a picture rather than a black frame....
It's a bit like trying to set fire to a match-head.. and NOT burn it all, but lea a pattern of un-burned sulfur in he ash!
NOW.... this is why times and concentrations get important.... remember, stronger the solution, less time the reaction needs.... BUT inside the tank, where you have dense regions on the negative that have had a lot of light, solution will come int contact with them and start to oxidise the halide quite quickly... BUT as t does so, it will deplete the concentraton of the solution.... so the reacton in THAT area will start to slow down..... meawhile in thin regions of the neg, there's not a lot of tckled halide, it wont be n such a rush to react, and so wont oxidise or oxidse so fast.. BUT the solution ear it will remain that much stronger and the reaction as fast.....
SO... if you didn't agitate, thin areas of the neg, you want to stay light would continue darkening, while dark areas of the neg you do, would slow down and not get as dark, and f you still pulled it from the soup before the picture had all turned black, the contrast would be deminished.
But, if you agitate, theory is that you 'mix' the solution, so locally depleted solution's dark areas, is washed away, and less depleted solution in light areas is mixed up with it to even out the concentration, throughout the whole tank, while at the same time, the ow uniformly weaker than t was solution, is spread over the entire film, and darker areas get solution that is 'stronger' than what was local to it, and the reaction carries on a bit faster, and lighter areas get solution that is weaker than it was, and the reaction continues more slowly, and you get better 'contrast' with more chance that darker areas will have fully developed, and less chance that light areas will have under developed.
Make sense? Oh-Kay......
Now... inversion.... principle is you make air above the film swap places with the solution beneath. In a spiral, you have a very small gap between coils of film, and a lot of chance you get air-bubbles 'stick' to the film... so, by inverting, you can make them move, and get solution to displace them. Meanwhile, as you invert, air has to percolate through the soluton, churing it up, making weak ad strong solution break up ad re-combine to mix it up, then go back to the film and cover the surface.
Twizzlg....you are only moving the film in the solution; what was at the to can stay at the top, what was at the bottom can stay at the bottom, you don't get the degree of relative motion throughout the solution, whilst worse, solution i those little gaps, between the layers of film, can 'stick' a t like treacle to a spoon, to the film surface and NOT get moved, or moved very much, ad much less mixed up.
Imagine trying to stir sugar into a cup of tea, by swirling the cup, rather than using a spoon.
Oh-Kay... there's no really right or wrong answer to whether to twizzle or invert... most correct is probably a bit of both... inversion, is definitely the more rigerouse and more reliable, and from chemical point of view likely to do much more.... so is probably the 'better' technique... I certainly find it so.... but remember we are trying to cheat the chemistry, and some-times it can be too much of a good thing, and mixing the chemistry up so well, you effectively keep the concentration up, and tend to over development, and you can get 'surge' where areas that get a lot of liquid washing past it get a bit more development than others. Relying on twizzling alone, you might not get the mixing, or movement, but not shifting so much fluid so far or swapping it with air, action tends to be more gentle, and you tend to need a lot ore of it to get the same amount of mixing, and there is risk, the viscouse atracton means you dont so readily shift air bubbles and the like.
So personally, of the two, I will invert two or three times, fairly slowly, when I first add solution to 'wet' the film and displace air, then in a 3 minute dev, with an agitation every 30 seconds, I'll probably 'invert' at prescribe intervals, with a quick 'twizzle' in between, just to give it that 'little' it of movement. Of the two I would rely on inversion as sole means of agitation, I wouldn't rely on twizzling as only means of agitation.
BUT... you have a leaky tank lid... so you are using twizzling only, to avoid spill.. ad so denyg yourself one and probably the better technique, and hence introducing possible dev faults for the sake of an equipment fault..... Washing up bowl! Use one! Do your inversions over the bowl to catch spill and avoid mess, and if needs, use extra solution at first fill to allow for any loss during inversion.... and (possibly or, at a pinch!) get a new tank.. they aren't usually all that expensive! Your leaky lid is a fault, and rather than fixing or compensating for that fault, possibly making more in consequence..... and you KNOW what this fault is, you dot have to diagnose it fro surge marks or dev banding or anything... so fix it and eliminate known areas of uncertainty! Its a easy ad obvious and simple one to fix, before you even begin.
