A detent turns a sliding post into a set position
The notches on a headrest post are detents: shallow stops that let a part move between deliberate positions instead of sliding freely. Inside the seat, a spring pushes a small pin toward the post. As the headrest moves, the pin rides over the metal until it drops into a notch. That click is the pin seating in a stop.
Pressing the release button moves the pin out of the notch, allowing the post to slide again. Let go, and the spring pushes the pin back toward the post. The arrangement makes adjustment easy by hand while resisting the headrest’s ordinary upward or downward movement.
Why the adjustment matters
A head restraint is meant to limit how far the head moves backward relative to the torso in a rear-end collision, which can reduce the risk of neck injury. Its benefit depends partly on where it sits: a restraint set too low may leave the head less supported when the seat occupant is pushed backward.
The pin is only one part of that system. It keeps the headrest at a chosen height; the posts and seat structure support it. The small click is therefore more than a convenience: it tells you the sliding part has reached a position where it is held rather than merely resting in place.
To change the height, the button temporarily disengages that stop. Once the button is released, a notch catches the pin again—so the headrest stays where you set it instead of creeping down with each adjustment or bump.
