Pre-exhaustion is one of the Weider principles and is a type of compound set that uses an isolation exercise followed by a compound movement.
The idea is to fatigue the muscle as much as possible using the isolation exercise and then work the small muscle further by using other large muscles.
For example, you could pre-exhaust your medial delts with side lateral raises before switching to the overhead press where triceps strength can be used to further punish the delts.
Other popular pre-exhaustion compounds are leg extensions and leg press for quads, flyes and dumbbell press for chest, leg curl and Romanian deadlift for hamstrings, and barbell curls and chin-ups for the biceps.
Whatever complex you choose it is important that these exercises are done with no rest in-between. That means you need to find a place in the gym where you can perform both of these exercises without any interruption at all.
If you haven’t used the pre-exhaustion principle in your training you are missing out on a highly effective tool. You will experience a burn in your muscles that will force your muscles grow.