no idea

@jonesmh Actually, whites queen is preventing b4. 19...Qd6 20. Bg5 Bxg5 21. Qxg5 b4 is really good for black.
19... Qb6 20. Bg5 e5 is even more fun.

Thanks to everyone for the help and suggestions.

@TheMightyNose I see the white queen protecting b4, that's why I said that black can't enforce b4.
In the second line, I suggested 20. Nb3 with a possible Nc5 to follow. Although a better idea may be to wait on this discovered attack and play a4 first. The knight might be better posted on b4 with lots of the a file squares covered. Black can always bail out with Be7 and Bxb4 with a drawn endgame.

@ jonesmh # 10
Just my opinion.
1a. Although I don't like Qd6 because there is a Q against Q file, 19...Qd6 20. Bg5 isn't so bad - rather, it allows Be5, setting up the battery on h2. Be5 forces f4, which does concede Black's dark bishop in a trade for the knight. However, it opens up the light square bishop, eyeing g2 long-term.
1b. Or @TheMightyNose #11. I agree, I like Qe7.
2. 19...Qb6 20. Bg5 is losing for white, to 20...e5 hitting the pinned knight. Further analysis required.

... I think I basically just copied @TheMightyNose on #14. My bad.

@AREA11
You aren't really copying me, what you intend to play against Bg5 is completely different. And you don't really like Qd6 too much, whilst it is the move I'd probably play.

@TheMightyNose If you say so.