Thanks for that detailed reply Vet.My reason for asking is that i have my own home gym,i train on a TITAN,and i`ve got quite a few items of equipment that i`ve made myself from different scrap that i`ve found on different job sites i visit.I`ve made myself dumbells from 20lb up to 150lb all from scrap steel i`ve found.Recently on a building site i worked on i found a suspension spring from a car,its about 18 inch long,diameter is about 6 inch,and i`ve been wondering what to "make" out of it.From seeing the spongbob squats i was thinking about welding the spring onto some checker plate i have for the base,and i`ve also got some 2 inch thick rubber,about 6 inch wide,that i could screw onto a piece of checher plate at the top.Then i could use it in the same way as the sponge,just to get a bit of "bounce" in the bottom of the squat.Thats why i was interested in what benefit the powerlifters got from using the sponge.