Transwave

Ensoniq used this type which is in fact a very very shot loop that covers no more than just one or 2 cycles of a wave within a sample which is more or less a transition form one extreme to another (different harmonics)

in fact it is a zero-crossing that takes one real period of one dute cycle of a waveform (shape!) and will be moved or advanced to the next period by keeping the distance (same frequency!), the loop lengths stays the same but the position will be advanced and moved to the next wave. you may use an envelope generator to do that job - so you can move (back and) forth - especially when a multistage env generator is used.

very close to: Wavetable Synthesis