Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6/5. O-O/5...Nxe4/6. d4/6...exd4/7. Re1/7...d5/8. Bg5

=Ruy Lopez, Riga Variation=

White's move threatens black's queen and attempts to eliminate the dark side bishop. Note that in the main line of the Riga Variation, that bishop is sacrificed on h2 for attacking chances on white's king.

8...Be7 accepts the trade, usually following 9.Bxe7 Kxe7 to break the pin coming from the a5 bishop.

8...f6 gets complicated. Now white has a piece in danger as well. White can respond Nxd4 and play can continue toward the main line. Simply moving the bishop again loses a tempo and the initiative.

8...Qd6 moves the queen out of the way, though it does take the square the bishop needs to sacrifice on h2.