This is what might be the first phasor-based envelope (correct me if I'm wrong). But basically I got the idea from this Cyclic Skew module by Pulp Logic http://pulplogic.com/product/cyclic-skew/

The envelope is triggered on a high gate and will go through its entire cycle before repeating. The envelope loops as long as the gate is held high. With different combinations of AR, skew, and gate width, you can get all sorts of rhythmic envelope sounds that still stay in sync with the music you're playing.

AR controls the speed of the envelope - you can go from 20Hz (fast) on the left down to realllllly really slow envelopes, until basically infinitely long on the right.

Skew controls the balance of attack and release times.

Left - attack = long, release = short
Middle - attack = release
Right - attack = short, release = long