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This is a standard way to reduce a framework based on 4435. The 3/4 exchange may sometimes be omitted.
Starting from the 3453 enclosure, after , rather than Black a, is possibly too steady (though one can hardly call it a mistake - it's one possible answer to this as middlegame joseki). This accounts for White's willingness to force Black in the reduction sequence.
Calvin: Another idea which is common in pro play is for white to invade at the 3-3 and then attach at . makes miai of a and b. If black plays a, white can make a position on the left side. If black plays b, attempting to seal white in, it is not clear white can live but the aji will be bad (see below).
See also : 3-3 point invasion query 6
Exchanging for gives White a big endgame play at 3. Black needs another move at a to secure the corner, so this should be played in the early endgame, when it ought to be sente.
If Black reacts on the other side to the marked play, White gains life in the corner.
The case in which White has stone at is important, because is often played in response to this approach. But even so, the invasion cannot live (Timm: strong player please check this).
After , there is no escape
tapir: A white stone at CC threatens to connect out.
After , a and b are miai for black. Note, however, that if white can play the hane at c in sente then white can live after that with b.
White can also choose to play outward at , allowing black to cut. That way, white will lose the corner, but may live along the top side.
This formation may arise from the 4435 enclosure, usually with at least one white stone at a or b. It is known as the Three Crows.
Even with both stones in place, the invasion at need not succeed. is something of a concession (see two diagrams above for the reason, in a related case), but the corner is then safe enough.