: ( 648aa)
White continues with the Main Semeai.

We will assume that White does not intend to block at in this subvariation.

:
The Main Semeai continues.

Black must not play any reduction.

Please note that Black is unable to give atari at , because this loss of one move would push him below the Punishment Semeai Liberty Level.
White would not answer with the "usual" move at , but take one of Black's liberties in the Main Semeai instead, and win the Punishment Semeai.

Even if she was so very kind to answer with , Black could not connect at , for the same reason as above.
After her decision point, White would capture at (and Black would recapture), getting the one point back that she lost by occupying the point of .

This implies that White will give atari with after her decision point, and Black will connect at .

Capture Variation:

Compared to the superordinate variation there is no change.

White + 2 => White + 2

Semeai Variation:

Compared to the superordinate variation there is no change.

Black + 4 => Black + 4