3-4 point low approach shoulder hit
in response to white's low approach is something of a trick play. If white responds correctly, black's result is slightly suboptimal.
Pushing through with is the obvious response. After
, white has two ways to cut, a and b.
Cutting with a is a mistake, and white will get a bad result. Cutting with b is correct.
Continuation
The normal continuation sees both players getting influence. Black keeps sente, but white's shape is somewhat thicker than black's shape. This is considered slightly better for white.
Later, a is large.
Falling for the trick
If white cuts with , play continues like this. After
, both players have influence, like in the correct continuation, but black's shape is thicker here. Also, due to the liberty filled by
, black now has a tesuji at a to claim a big corner.