*pulse is an arpeggiator midi effect which continuously generates incoming midi notes. The notes can be arpeggiated as single notes or as polyphonic structures, i.e. chords.
The speed can be varied by an envelope, which is running over a specified amount of time as one shot or in loop mode. An additional "idle" speed can be set as a fixed value if the envelope is not triggered. Further documentation can be found in the Max for Live device if Ableton Live's Help View is turned on.