How does a head coach start a challenge?

Under Rule 14 Section II, the challenging team must take these steps in sequence:

1. Call a legal timeout immediately after the call in question (unless a mandatory timeout or a timeout called by the opponents is already under way). "Immediately" means before the ball is given to the thrower-in, given to the free throw shooter, or tossed on a jump ball.
2. The head coach signals for a challenge with the instant replay signal (twirling an index finger) toward the officials, during the same interaction in which the timeout was called.
3. During that same interaction, the head coach tells the officials the specific event being challenged.

If the call came just before a mandatory timeout or an opposing team timeout, the head coach has up to 30 seconds from the start of that timeout (measured by the timeout clock) to signal and name the event.

A team loses its chance to challenge a call if it is called for delay of game after the call in question but before it calls the timeout and signals for the challenge. Once a valid challenge triggers review, the official notifies the official scorer, who records the challenge and its outcome.

Source: 2026-27 Official NBA Playing Rules, Rule 14 Section II.