The Rubber Sheet (also known as the Rubber Trampoline) is a block added by IndustrialCraft 2 (IC2). It causes mobs and players to bounce when they fall on it, negating fall damage. There must be a one block space underneath the Sheet and it needs to be connected to blocks on at least two sides for it to function. The mob or player's bounces will gradually decrease in height until they come to a stop. If a mob or player falls from heights over 11 blocks the Sheet will break.
There is another form of Rubber Sheet which has not been fully implemented yet called ic2.blockHarz in game, but is referred to as "Sticky Sap Sheet" within the code. Originally this was placed by right-clicking with Sticky Resin and would reduce fall damage if landed on. It must have a block underneath it to be placed but does not require any blocks around it unlike the Rubber Sheet.