Here is a diagram of a classic barging situation. The boats are approaching the mark (which is a signal boat) to start.



Rules 18 and 19 do not apply between Blue and Yellow. Yellow is not entitled to room at the mark. Yellow is obligated to keep clear of Blue by rule 11.

Blue is not changing course in this diagram, so rule 16.1 does not apply to her. There's no room for Yellow between Blue and the mark, so she has to turn away.

And here's a scenario where the leeward boat "shuts the door" in time.



At positions 2 and 3, Blue changes course and is obligated by RRS 16.1 to give Yellow room to keep clear. Yellow responds promptly, meeting her obligation to keep clear of Blue, but has to turn away to miss the mark.

Here I've drawn Blue shutting the door on Yellow in time, but Yellow goes below the mark anyway.



Yellow breaks RRS 11. Blue takes avoiding action as required by RRS 14 and protests. Blue should win this protest.

I hope that helps,
Eric