Originally Posted by Peter Dell'Orto
At the bottom of the squat, deadlift, bench press, etc. the resistance is lighter. As you move the bar up, the resistance gets heavier - either the weight of the chains increases as they come off the ground or the bands resist more as they are stretched. So in order to deal with this you have to move the bar explosively; if you lift slowly you'll hit a wall of resistance and stop. Instead you lift the (lighter) bar up from the bottom fast to help get past the increasing resistance at the top, when it's heaviest.
this is basically it. "accomodating resistance
" as your leverage gets better, the weight on the bar increases. it's really helpful for dynamic efforts where you are trying to accelerate the bar. also works great for deloading a movement so that the bottom is less stressful and the lockout is very heavy.
the amount if resistance added with chains has a fairly linear feel, the weight added from band tension can be staggering. just unracking a bar with 300 pounds of band tension is nasty.
conversely you can also hang the bar from bands (my fav) so that it takes maybe 70-200 pounds off the bottom but takes virtually none off the upper 1/3rd. this is a great method for learning how to hit the hole in a squat or to pause a bench or pull quickly.
in one article louie simmons called this the "future method" where you put your goal weight on the bar and squat it for reps using suspended bands to get used to unracking and initiating a squat with a weight that's maybe 150 over your true max.
there are a number of opinions on whether accommodating resistance is really useful for raw lifters. Personally, I don't use gear but I've found that bands especially can be useful for overhead pressing and jerks; chains are great for deloading front squats. So it's not solely a PL thing.