When two or more notes are played on a monophonic synth, or a synth that is in a mono mode, the synth makes a decision about which note will sound according to its note priority algorithm. However, if the performer subsequently releases this note while holding others down, the synth must choose a new note to play from among the held notes; this is called a retriggered note. The synth faces a conundrum as to what the velocity should be applied to this newly chosen note, since the performer hasn't actually struck another key. Some synths will use the release velocity of the released note as the velocity of the new note.
An envelope generator which can be made to return to the attack phase by a new trigger or gate signal, even though it may not have completed its cycle from the last played note, is said to be retriggerable. Not all envelope generators are retriggerable; some insist on progressing to at least the beginning of the sustain phase before they can be made to start a new cycle. A few envelope generators are switchable as to whether they will operate in retriggerable mode or not. Typically, retriggering moves to beginning of the attack phase; in some envelope generators, retriggering causes the attack to start from the current level.