The first two points are correct. Barging most often happens when a boat (W) sees what looks like a nice space to leeward of the signal boat, and to windward of the nearest racing boat (L). That space usually goes away quickly as the clock winds down. A good crew on L will not let W in there, but the only legal way to keep W out is to luff with enough time such that W can't get overlapped with the stern of the signal boat. If that happens, L has allowed W space to which W was not entitled, but at that point has no recourse (especially not luffing W into the signal boat).
At least, that's how I understand it. There are rules and cases if anyone wants to do a little digging...
