What pre-exhaustion is

You take an isolation movement to momentary muscular failure, then move at once into a compound that works the same muscle. The target is already tired, so it — not the assisters — sets the endpoint of the working set.

How to pair the movements

Match an isolation to a compound that share the prime mover: a leg extension into the HIT leg press, a fly into a press. Keep the handoff immediate and the loads honest.

Why HIT uses it

Pre-exhaustion is a way to make one set count when a straight set leaves the target under-worked. Fit it into a considered plan rather than bolting it onto everything — see designing a HIT program.