chess online

After
1. f3 Bxf3
2. Qd2

Things look okay for white.

Now consider 1...Bc5. If white want to try to keep the queen on the board, he must play 2. h3 or 2. h4, and a possible continuation is

2. h4 Qf1+
3. Kh2 Qf2+
4. Kh1 Bd5

And if the white queen ever gives up control of f3, Bxf3 is mate, so essentially black can force the white queen to sacrifice herself for the bishop in order to prevent mate.