What vet said; and only your imagination limits their use, (chain and bands)
eg:Some guys hang them over the cage and suspend the barbell in the heavy bands....
Ok, when you press this, you have a boost @ first, but it gets heavy real FAST! So, again, we learn to increase velocity out of the bottom....
If you reverse all the ways you've learned to make a light weight feel heavy, you'll lift more and IMO hit different muscle fibres.
I dont know if thats the technical/ scientific explination, but just handling that heavier iron w/ increased velocity at the start, to the point of near stall does something different to the muscle.
Elitefts has lots of band set-ups and chain video clips on that U-Tube page of theirs.
I like the feel of the chain as it builds up, Im gonna hit my local Tractor dealer sometime for some bigger stuff.