Bottom bondage
From wipipedia.org
Bottom bondage or buttock bondage is the placement of ropes tightly round the base of each buttock. The idea is to make the bottom bulge out, either for aesthetic reasons or to prepare the bound person for spanking or caning.
It is difficult to get the ropes to stay in place. A fixing rope round the front, across the groin, is usually needed, and the bound person should stand upright or lie flat and not be allowed to bend at the waist.
As with breast bondage, this technique does not immobilise the bound person.
