Actually, the horde can win even without backcapping. They can defend the two FW Keep towers instead. This also defends Drek, so it prevents the alliance from trying a "Drek with two towers up" strategy.
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Defend the FW Keep towers and win fast. There is no good reason for the horde to not defend.
"There is a pervasive myth that making content hard will induce players to rise to the occasion. We find the opposite. " -- Ghostcrawler
"The bit about hardcore players not always caring about the long term interests of the game is spot on." -- Ghostcrawler
"Do you want a game with no casuals so about 500 players?"
Choke points do not really have much effect in wow because of the game engine.
1. players can run through each other
2. attacking requires targeting
Even though there are abilities that do not require targeting, they are not really common and are not effective at stopping 40 people from running past.
Alliance could just run through hordes and proceed to kill the boss.
Except the alliance doesn't send 35 people to FW Keep. A small number of paladins and DKs dribble in first, then some more, but not 35 overall. 10 or so are behind guarding IB/TP, and more may be back capping. The early arrivers are crucial for the usual alliance strategy, to get the cap timers ticking on RH and the FW towers, and are easy to kill. The defenders are assisted by the archers, who are stronger now, and by the map change that closed the back way into the upper keep area behind the west tower.
Alliance can't really push into the upper keep until they have a nearby graveyard, since the horde is rezzing right there. Any alliance casualties are taken out of the fight for a long time, spawning back at SH GY or farther north.
Properly executed, and with the usual alliance response, this can lead to the horde zerg beating the broken alliance zerg. No turtle is needed, although one may break out.
"There is a pervasive myth that making content hard will induce players to rise to the occasion. We find the opposite. " -- Ghostcrawler
"The bit about hardcore players not always caring about the long term interests of the game is spot on." -- Ghostcrawler
"Do you want a game with no casuals so about 500 players?"