Checkpoints, in the military, are used to track progress of a unit’s movement.
In Lean, checkpoints can be used in a similar fashion. Checkpoints can be linked to specific process steps. When the sequence of work is standardized, the operator should hit those checkpoints with the same time remaining in the takt time each and every cycle.
Simply put, if an operator knows that at Step 3 (her checkpoint), she should have 3:08 left on the countdown clock, and she only has 2:07, she is behind. If it reads 4:11, she is ahead. Both situations indicate a problem of some sort.
Hitting checkpoints with too little time left means a potential line stop. The operator should call for help.
Hitting checkpoints with too much time left normally indicates a missed step. The operator should check her work.