In duration modulation, a track's note durations are altered by one or more modulator tracks. A duration modulator track is interpreted as an array of signed offsets (in MIDI ticks), which are added to the note durations. If a target track has multiple duration modulators, they're summed. The duration is prevented from being less than one.
Decreasing the duration causes the notes to become more staccato and percussive, whereas increasing it causes the notes to become more legato and potentially overlapped. Duration modulation is more effective if the output instrument has a rapid attack and decay.