Bound is a "new" feature that was introduced in Tekken 6. While the distinct properties of the Bound system is new to Tekken, it was based on an already existing system of juggle finishers present in Tekken 5. Some juggle finishers would literally "slam" the opponent into the ground, which meant that they could not tech roll.
The Bound system builds on that by making the lower body of the opponent "recoil" when they slam into the ground, partially lifting it into the air, giving the player the chance to continue their juggle. Additionally, in stages with breakable floors (Anger of The Earth, Cemetery and Fallen Colony), if the floor is broken at any time, the fighter that was knocked through the floor will have their body react in the same way.
In some reported cases, it has been said that an opponent has been bound more than once, though they have yet to be verified, and the circumstances are not yet investigated. It is likely that these instances have been misunderstood by viewers where a player has "picked up" their opponent in the early frames of getting up, giving the appearence of a single juggle, when in fact, it is two seperate juggles (which also means the bound counter was reset).