You can reduce the leverage during an exercise in two ways:
1. Altering the position of the body through changing the progression (i.e changing the centre of mass from straightening out the legs in a front tuck lever on the rings for example).
2. Changing the length of the muscle (eg. performing a press to handstand or a front lever with straight arms is much harder than doing the same skill with bent arms).
By training the correct progressions (which help build the correct strength), we are then able to progress into a more challenging variation (i.e reduce the leverage further), putting more force on the muscles to contract and work harder - this helps build strength and muscle mass over time.
When the central nervous system is shocked (via the appropriate strength training exercises) and senses the recruitment of all motor units, it further increases strength by sending more rapid signals to the muscles to tell them to contract faster (under the increased force that's applied) - which again helps build strength.