When do I use the red flag for improper entry?

Under World Aquatics rules (Part Six, Articles 3.11.2.4 and 11.1.6), the secretary signals with the red flag (or another approved method) for:

• Improper entry by an excluded player or a substitute, for example re-entering before the exclusion has ended, or from the wrong place. The signal stops play immediately. The secretary may signal on their own observation, or after an assistant referee raises both arms.
• The Referee's award of a player's third personal foul.

After an improper entry, the other team gets a penalty throw, taken once the offending player has been removed from the field (by immediate substitution if needed). An excluded player who came in early must leave before the penalty and may re-enter after it at the earliest of the exclusion expiring or their team regaining the ball or being awarded a free throw, goal throw or penalty throw. If the exclusion was for violent action, the substitute may enter only when the four minutes are up.

Entering during play without the correct cap also counts as improper entry.