Deliberately focusing on the target muscle during a set, which measurably increases how hard that muscle works at light to moderate loads.
It is real and it is narrow. Studies using EMG show that an internal focus ("squeeze the chest") raises activation in the target muscle during isolation work at moderate loads, and that the effect fades as the weight approaches maximal, where the body recruits whatever it needs regardless of what the lifter is thinking.
So use it where it works: isolation exercises, machine work, and the higher-rep accessory sets, where a client rowing with their biceps can be taught to row with their back. Use an external focus instead on heavy compound lifts, where "drive the floor away" outperforms "squeeze the glutes".
A practical route in: one set at half the working weight with a two-second squeeze at the shortened position, then the working sets.