Though this might look like a traditional envelope, it is not.  The Spline Envelope has two modes, switchable with the Mode button: Timer, and Slew.  In Timer mode, a gate ping restarts an internal timer, which is multiplied by the Timer(x) knob.  For one gate ping in timer mode, the spline goes from start to finish.  In Slew mode, however, a gate ping is fed through a slew limiter, controlled by the Slew knob, and the spline moves from low to high and back to low again, crossing the spline forwards and then backwards.  The strength of the envelope is controlled by the Pressure knob, which works from a value of 0-2.  The higher the Pressure, the louder the envelope gets.  The Zoom knob helps adjust the range of the waveform meter behind the spline.

At high slew values, quick pings will gradually push the envelope all the way to the end a little bit at a time.  Try this with a sequencer setup.