Second Line Clamp

Clamp on the second line  

The clamp on threatens to watari underneath with a or start a second line capture with b.
Depending on the context (hinted at by the circled points), Black can either resist at a, connect at b or even play c (but this leaves in atari after white a.) If Black can't resist, the clamp is a big endgame play.

Can black resist?  

To resist black needs to capture directly or in a ladder. A net is not enough since + will be captured then. (Alternatively, if is very stable - more than in this diagram -, black may force once or twice and simply capture .)

Less direct White moves  

Another thing to keep in mind when trying to resist with is that White can just threaten to play a with or for example. Such moves can often carry other threats too. All things considered black has to be very careful about resisting with .