Its an easy trick to do. They just take the video of the carpet before they place the block under the leg to make the indentation, then edit it as if it was after the ice has relaxed the carpet back.
Heat and cold can help change the memory, and recover the shape/lay back on fabric and carpet in some situations, but heavy indentations in carpets (and mainly underlay) will take many months to recover its memory back, if ever at all.