If it were white to move, the hane at the triangled point is the shape move. Black's hane would be vulgar, so I think black can play at the triangle square himself, or make a 1 point jump. Regardless which is better - black "owes" a move here for local shape.
What happens if black already has the following marked stone:
It seems like black can just tenuki here. But then
is sente for White (I think), and a to be it looks like an inefficient, silly shape for black. What did White lose? Well the sansan invasion, but when White chose the footsweep he already stengthened Black, so that probably wasn't White's plan anyway. The approach on the upper side is still possible, as is a 2-4 invasion. Seems like White just strengthened his side for free.Instead if Black shouldn't tenuki, a 1 point jump seems right. but it's still kind of overconcentrated, and the footsweep becomes sente.
It makes me wonder why I don't see more of these footsweeps after Black plays the original 3rd line move to threaten White's side. Intuitively I feel like I am missing something, but I can't see it.
