As it says on the Tin. It must be an Enhancement Skill of your making. Simple enough.

Here's mine:

Release Restraint — By initiating this ability, I gain a significant increase to my capability that is being enhanced for a third of the time I was restrained total. Time before ability activates can stockpile.

Simple example: I wear heavier than normal shoes? Maybe steel toes? I can pull a Rock Lee and run faster than normal by use of this ability, but enhanced by Nen. I do this for a total of 24 hours? I got 8 hours of that speed.

Advanced example: By having a form of mental disability, like developmental delays, using this ability allows for me to go giga-galaxy-brain for a third of the time I've had this mental disability. — Pretending I have two mental disabilities, so if I were to have something like FAS or trauma severe enough to suppress memories, I stockpile time even more for those disabilities. — These disabilities don't go away, so for every 3 units of time, I gain 1 of that unit back. And on top of that, the ability can be turned on or off at will, so because of that, I can use a standard 16 hours conscious, 8 hours unconscious, and have that turn into a case of functionally spending only half as much time. — Yes, disabilities are restraints.

Super advanced skill use: Knuckle can help you spend 30 days prepping for a big fight. Because going bankrupt also restraints you severely for those 30 days. Now imagine the Nen output available if you just flash Release Restraint usage. — Imagine the stockpile from Illumi's brain needle too!