The mechanism is simple: when an action feels chosen, connected to your own values, and gives you a visible sense of getting better at something, you keep doing it without needing to be pushed.
External motivators like bonuses, praise or deadlines can start a behaviour fast, but they tend to fade the moment the reward disappears or the deadline passes.
Here is the honest arithmetic: if you only show up for a habit when there is a reward attached, you show up on reward days and skip the rest.
If even one part of the habit feels worth doing on its own, like the calm after a walk or the satisfaction of a tidy inbox, you now have two reasons to show up instead of one.
Stack a few of those internally-satisfying reasons together and the habit starts surviving the weeks when nobody is watching and nothing is on the line.