if you press the third button from the left, if you press it once, it'll show a box with a flash sign and a H. this is high speed shooting it'll recharge the flash quicker before the next discharge.
And wow, never knew what that multi setting did before, i do now
"High Speed Shooting", as you call it is actually "High Speed Sync". It has nothing to do with allowing the flash to recycle more quickly. It enables you to shoot with a shutter speed higher than your normal flash sync speed. On a Rebel series camera the max sync speed is 1/200. On most cameras higher up the food chain the max sync speed is 1/250.
High Speed Sync allows you to shoot with any shutter speed of your choice, right up to the camer'as maximum, be that 1/4000 or 1/8000. It changes the way the flash operates completely and is actually quite inefficient, meaning it sucks more power from your flash and actually increases recycling time.
I would typically use HSS when needing to use flash in daylight, to fill some shadows, but wanting to retain a wide aperture for shallow DOF and nice bokeh. Without HSS you would be limited to a shutter speed of 1/200 or 1/250. In bright sunshine you would need to shoot at around f/10 or f/11, at 1/250 or 1/200, even at 100 ISO, just to avoid overexposing the overall scene. Those are not great aperture choices for a nice portrait. If you wanted to shoot at, say f/4 or f/2.8 then you would need HSS so that the flash could operate with the higher shutter speed necessary to maintain a good exposure for the ambient light conditions.
I base the figure above on the Sunny 16 Rule, which says that in bright sunshine, with f/16 as your aperture, your shutter speed should be the reciprocal of your ISO. So if your ISO was 100 and your aperture was f/16 your shutter speed would be 1/100. If you used 200 ISO your shutter speed would be 1/200 etc.. So for a standard exposure in bright sunny conditions (no cloud or haze), at 100 ISO, you could choose combinations of aperture and shutter speed as follows....
f/16 and 1/100
f/11 and 1/200 - max sync speed for a Rebel and a 5D
f/10 and 1/250 - max sync speed for most other camera bodies. a 1D3 can sync at 1/300.
f/8 and 1/400
f/5.6 and 1/800
f/4 and 1/1600
f/2.8 and 1/3200
f/2 and 1/6400