A quote from http://www.the-digital-picture.com/Reviews/Canon-EF-100-300mm-f-4.5-5.6-USM-Lens-Review.aspx
"The zoom ring, on the other hand, is too easy to use. The Canon EF 100-300mm f/4.5-5.6 USM Lens will extend all by itself when hanging downward. "
Same thing happens on the 100-400L, which is a push/pull zoom. Is this one push/pull?