Ah, thank you, I think I see now. If black knight takes g5, then black loses both rooks while white loses only one. Overall, white gains the pawn and a rook.
I think I was confused that black's response of Ng7 moved off the pinned file and so seemed to ignore the pin. Now I see that by moving to defend the e8 rook it ensures all rooks are exchanged and so limits white's gain to only the pawn.
Ah, thank you, I think I see now. If black knight takes g5, then black loses both rooks while white loses only one. Overall, white gains the pawn and a rook.
I think I was confused that black's response of Ng7 moved off the pinned file and so seemed to ignore the pin. Now I see that by moving to defend the e8 rook it ensures all rooks are exchanged and so limits white's gain to only the pawn.