When do I keep my flag up after the ball is thrown in?

Normally you lower the flag when the ball is thrown in. Keep it up (Law 6.26c) when:

• the thrower steps into the field of play as part of the throwing action
• the wrong team threw in
• at a quick throw, a different ball was used, or the ball was touched after it went into touch by anyone other than the player who carried it into touch or the player throwing in

A quick throw is also disallowed if a lineout had already formed (Law 18.5); a quick throw must go between the mark of touch and the thrower's own try line, parallel to or back towards it, reach the 5 m line, and be taken with both the thrower's feet outside the field of play (Law 18.4).

World Rugby guidance: tell the referee verbally, or keep your flag up, when a quick throw should not be allowed. Whether the throw was taken from the correct place is the referee's decision, not yours (Law 6.26d).