If 18. ...B/Nxe7 then 19.Qxd7, winning Q with no compensation. Whatever move he makes is going to lose the Q. Therefore, 18. ...Qxe7 19. Rxe7 Bxe7 . He is only slightly down in material ( Q for R and N). Had I moved Nf6! I would effectively win Q for N. Hope that makes sense.
I was playing someone over 100 rating points higher, and thought i could never win. I tried to keep the initiative, and it proved winning.