3-3 point invasion query 9
tderz: It is a method for Black to get sente.
 Important is the stretch . If crawls, Black does not have to respond immedeately locally and gets global sente.
The cutting point at c is protected, because Black has also more liberties (around d).
The more "usual"(?) variation on the right ends in gote after white a, Black b.
Having checked gobase, I get the impression, that the continuation following my reasoning is played least often.
White playing d herself, with Black replying at e occurs often, as does the simple (?) Black f for (=unkx80's variation below).
My summary could be: if White crawls with – take sente.