[1]
The assumption is that this is made from the initial 4-4 point.
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.
The 3-3 invasion in this position cannot live, but may be useful in other ways. Sometimes it can function as a probe. Here it 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.
[2]
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.
With just in place, Black can try
(or Black at a) in reply to
. The feeling is then to avoid being forced, and make
into mochikomi.
That is at the cost of some bad aji here for Black. In pro games can be played as a probe.