They go back to the box.
If, at the start of any beat, the amount of Force left in the pool is less than the amount that should be drawn, draw all remaining tokens from the pool and gain the amount extra you need from the box. The game ends at the end of that beat.
The game should last between 12 and 18 beats, unless someone runs out of life before then.
Fix: the game ends at the end of the next beat.