Telescopic slides with self-colse or soft-close

What makes telescopic slides with self-closing or damped self-closing? 

A damped self-closing mechanism is a damper that ensures that the runner is slowed down when the drawer is pushed in and retracts slowly and silently instead of slamming hard. You can push the slide closed carelessly, so to speak, and it slows down and then closes itself. A so-called soft-close is an insert that can also be retrofitted to some rails.