Everyone knows that at sea, a collision heading can be defined by noting whether the relative bearing of the other vessel remains constant. Out in the open, there is time to take compass bearings or wait to see if a ship moves along the guardrails from a fixed helming position. Inshore, it’s a lot easier to line up an approaching craft against her background. If she remains stationary, you will collide. If she’s moving, you’re OK. A ship ‘gobbling’ up ground behind her bows is safe, so long as she is spewing it out from her stern as well. If both bow and stern are munching into their backgrounds, the bit that isn’t moving is somewhere between them and you’re going to whack her amidships. The same goes for a moored yacht when you’re short-tacking up a river!
This proposition always generates bar-room dissent, but racing yachts use it, so it can’t be all bad. Just don’t expect miracles when things get very close.