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A lightweight version of the classical block, prefect if you have to carry it with you between practices.
With the correct material choise the block gives good support, and still has a little "bounce" so it is comfortable to use even during longer sessions.
Another positive thing is that it doesn't matter if you drop it due to it's light weight.
Often used in iyengaryoga , yinyoga and even during astanga "playground"/technique lessons.