If I remember correctly, they either just sign the scoresheet or stop the clock first and then sign the scoresheet to indicate that they resigned. There is no handshake, and they do not speak to each other since the WCC2006 controversy.
Here are two examples:
Candidates Tournament 2014, Rd. 13 (Kramnik-Topalov 1-0),
Norway Chess 2014, Rd. 6 (Topalov-Kramnik ...
Well I'm no GM but for me there's a couple of things going on.
In the given position white is threatening to take the pawn on a6
If white can get a knight to d6 he will fork the rook on e8 and the bishop on f5, so winning the exchange after a move like Bd7
Thus black's move 23 protects the pawn on a6 by uncovering the line of the rook on a8. White then ...