There's several different ideas behind it. Louie Simmons group (Westside) has been doing it for a long time but I believe they've moved to bands more than chains for their dynamic effort but still use chains for various things.
Cliff notes version is to use sub-maximal weights working to move them at max speeds.
Weight on the bar at the bottom of the squat (or bench) should be in a 50-60% range (most of chain resting on floor). If someone squatted or benched those light weights (50-60%) at max speed they have to slow down at the top of the lift....with chains/bands the resistance increases through the range of motion and the lifter can continue to try to accelerate the bar. Again, that's just a cliff notes version....you can research Westside's Dynamic Effort days.
My end goal isn't the same as Westside but I'm a big believer of incorporating dynamic effort in.
