See this thread: The MMC Thread (Mind Muscle Connection)
For me, as an example: I can do 70+ push-ups in a row before I get bored or too tired. Conversely, if I try to squeeze my chest, arms, shoulders and upper back as hard as I can, each rep, I only get 25-35. It’s because the goals is to get as much blood to the muscle vs. efficiently completing the lift.