Henneman was an OG when it came to explaining the neural adaptations that occur when we get stronger and more powerful.
The Henneman Size Principle (1)
When we perform any movement (concentrically)*, our nervous system recruits and activates motor units in a smooth stepwise fashion. Starting with the smallest type I motor units working its way up to the biggest type II motor units.
In that order. If a movement has a low enough intensity then only a fraction of our available motor units will be engaged for the task.
This evolutionary wiring system serves a few key purposes. Firstly, it allows us to execute fine motor skills with precision, and secondly it helps conserve energy. There is no advantage to going around all "Hulk Smash" every time you are washing the dishes or trying to type out an email.
Henneman and training
So as you try to lift progressively heavier and heavier weights your body will progressively recruit more of our total motor units to lift the weights. This is why for more experienced lifters you only really start developing strength above 80-85% of your maximum intensity. Anything less is just too cruisy. It's also why ascending warm up sets are a great short and long term strategy for performance, they prime our motor units by progressively overloading them without ever trying to smash them with a maximum effort cold.
The same is true for trying to lift faster.
The more explosively you move (or intend to move) the more your body will recruit these bigger, stronger, type II motor units.
The third way to recruit our type II fibres is through fatigue. The more reps we complete and the closer we go to fatigue the more our larger type II fibres are effectively forced to engage as we exhaust our TI motor units (2).
Be warned though, taking too many sets deep into fatigue can burn an athlete out, and lead to extra type I muscle fibre hypertrophy. Less than ideal for athletes needing to maintain a high power to weight ratio.
*Side note: Eccentric muscular contract behaves differently to concentric, being more energy efficient and not necessarily obeying the same stepwise contraction laws of henneman's principle (3).