Rule 17 - Action by Stand on Vessel

Applies when 1 of 2 vessels is to keep out of the way the other SHALL keep her course and speed.

The latter vessel MAY however take action to avoid collision by her maneuver alone, as soon as it becomes apparent to her that the vessel required to keep out of the way is not taking appropriate action in compliance with these Rules.

When, from any cause, the vessel required to keep her course and speed finds herself so close that collision cannot be avoided by the action of the give-way vessel alone, she
SHALL take such action as will best aid to avoid collision.

Let the other vessel take appropriate action.

If the give way vessel fails to act, the stand on vessel MAY take appropriate action.

Stand on vessel does not have to take appropriate action until she finds herself so close that collision cannot be avoided by the action of the give way vessel alone
This point is known as "In extremis"

only at the time of in extremis, is the stand on vessel REQUIRED to take action

A power driven vessel which takes action in a crossing situation in accordance with subparagraph (a)(ii) of this Rule to avoid collision with another power-driven vessel shall, if the circumstances of the case admit, not alter course to port for a vessel on her own port side.

This rule does not relieve the give-way vessel of her obligation to keep out of the way.