i don't mean to hijack, but i wouldn't know how to word the question in the search or in which part of the forum to look. anyways i don't understand the foil, this is the second time i've seen this and i'm curious as to why an airlock isn't being used.
I have heard from sources such as Chris White from White Labs that even the slightest pressure on the yeast during replication can have an affect on the yeast (I have heard as low as 2 psi). When an airlock or blowoff tube is used there is a slight pressure build up. I generally leave the foil on until the first signs of high krausen then switch to a blow off to prevent a mess. This yeast just took off.
If you watch brewing TV Dawson did an open fermentation experiment with a hefeweizen with an open bucket vs a closed bucket and side by side the beers tasted different. The only real difference that I can think of which explains these flavor differences would be the lack of pressure from the lid.
On a more realistic process side I also use the aluminum foil because it's easy since I am constantly going in and out of the carboy in the first few hours. I tansfer and then, if not at the proper pitching temp might stick in the fridge to cool a little more, then I will pull the foil off and oxygenate, then I might pull the foil off to pitch yeast, maybe I forgot a gravity reading so I need to pull the top off again.
Each time I go to pull that top off I spray that foil top with alcahol and flame the crap out of it with a creme brule torch - try doing that to your carboy cap! So there is also a convinience factor that probably is more significant to me than the osmotic pressuse nonsense I was preaching before