The technique of inserting a signal into an audio track.  Placing the recorder in and out of record at specific locations in the track.

Punch-in is a useful technique for correcting performer mistakes or inserting material into already recorded tracks. 

Have the player monitor the pre-recorded material on headphones to avoid track leakage.  Playback the recorded material some distance before the punch in point.  At the appropriate place in the track punch in the record button on the recorder for the player to record the new material.  Punch out of record as soon as the player has finished the new material or when the previous part needs to be back in. 
This takes practice! 
Some recorders provide a rehearsal mode which mutes the track to be recorded but does not actually record anything so both the engineer and player can practice their moves.