Crushed by the Atmosphere?

Reader Bronwyn from New Zealand offers this puzzler from down under:

When I was young being able to make a great sponge was almost as much a mark of proper womanhood as making a good Pavlova. (Thank goodness those days have gone.) The sort of sponge I mean is this: http://www.foodlovers.co.nz/forum/read.php?3,86688 with no fat except what’s in the egg yolks. They tend to sink as soon as you take them out of the oven, as you can probably tell by the proportion of egg to flour/cornflour (cornstarch to you, I think). To stop this happening you whip each cake out of the oven and immediately drop it (in its tin) onto the floor. Sounds appalling, but it works – much to my childish disappointment, because as soon as my stepmother discovered the “secret” I stopped getting to eat all the failed sponges. My record was two in one day, and she got so pissed off with me eagerly waiting for the next that the third went to the birds. The fourth was dropped on the floor (she finally decided to try it) and from then on there were no failed sponges.

My Dad’s explanation sounds as good to me as anything, but I was wondering how you’d explain it. Dad thought that what was happening that the bubbles in a failed sponge were more being sucked flat by the cooling air/steam inside them than collapsing, and that when you dropped the sponge on the floor the bubbles popped, letting cold air in to set them and stop them from flattening. The only issue I have with this is the bubbles right on the inside of the cake, which are quite a long way from the cold outside air.

Btw, the sponges, when made, are then sandwiched together with raspberry jam and lots of whipped cream and the top is sprinkled with icing sugar. If you want to be really fancy you put a paper lacy doily on top before sprinkling the icing sugar to make a pretty pattern. You can’t eat it with your fingers, you need a “spork”. To this day a variant with sliced bananas between the jam and cream is still my brother’s favourite cake.

Bronwyn insists this is no arcane secret, but something well known among home bakers in in New Zealand. If I had to speculate, I’d say that Bronwyn’s father might well be right. It seems at least possible that a bubble that’s been enlarged by expanding gas and steam might be pressed flat as structure cools and the steam condenses back to water. It becomes a little vacuum, in other words. Dropping the cake might open small cracks in the crumb that allow air to rush in and fill the void. Hmm…anyone else have any ideas?

UPDATE: Read Todd adds:

[My] dogs would love it if I started dropping food on the floor on purpose.