The detent ball is a tiny hardened ball set into the lock bar or liner that drops into a small dimple in the blade tang when the knife is closed. It is what keeps a folder from falling open in your pocket, and it is what makes a flipper work.
Closed, the ball sits in its dimple and holds the blade shut with a measurable amount of force. That is the entire mechanism preventing a modern folder from opening in a pocket.
Opening, the ball has to climb out of the dimple. That resistance is what a flipper tab works against: pull the tab, the ball pops out, and the blade snaps open. A weak detent means the flipper will not fire.
If a flipper feels sluggish, the detent is usually the cause: either the ball has worn, the dimple has deepened, or the pivot is over-tightened so the blade cannot accelerate once released.
If the blade opens too freely in the pocket, the detent has weakened and the knife needs attention. That is a safety issue, not a preference.