The clamp at is a tesuji for expanding eye space.
at
is captured with
at
.
may be postponed in practice - White has another option of playing at
.
can be at a.
cut is an unnecessary sacrifice. To save the three stones White has to play at
. Subsequently
and
are a tesuji combination, and Black lives with
. Now hane at a is gote. Caution: Do not play
at
: White can connect at
and Black cannot make the eye on the top.
(For advanced readers: at
is still better for White. Black captures at
, White recaptures - see the diagram below)
Black lives with a and b miai. Now White's first line is better protected on both sides than the Solution diagram above. The combined benefit for White is larger than Black's gain on the third line.