We have practiced the tripod group at our club. In response to , is the intuitive move for many players. We were wondering why this would be "wrong". I think that under these conditions, with all that open space around, this is actually a good move, but the tripod often appears in enclosed space, so that a white attachment at a becomes dangerous. Moreover, if there were an escape route along the left side, White would have blocked at instead of . So, at a is the main line.
So, Black escapes, but loses his corner territory and, what's more important, his eye potential there.
fractic: Of course black only dies with a bent four if he forgets to throw in with . Black can play a next for oshitsubushi even if white takes an outside liberty like b. And if white connects at then black can capture and still get an oshitsubushi. This is a transposed version of a line that's already shown on the tripod group page.
Herman: Right, good point! Commented too quickly :)