OK 13.Bg3!? interesting and Black's strategy needs to be repaired. Maybe, 13...Bxg3 14. Nxg3 Bd7 playable, but now I prefer not playing Ng4.

About the idea: 12..Ng4 13.Bg3!? compare with the line: 12.Bg5 Qc7 13.Bh4 Ng4! 14.Bg3 Bxg3 15.Nxg3 e5 16.dxe5 Ncxe5 17.Nxe5 Qxe5= thus
I will play (but I must check it) 13....Bxg3 14.Bxg3 e5

Just something that has me slightly confused in the (ultra) main line after: 1 e4 e6 2 d4 d5 3 Nd2 Nf6 4 e5 Nd7 5 Bd3 c5 6 c3 Nc6 7 Ne2 cd 8 cd f6 9 ef Nxf6 10 Nf3 Bd6 11 o-o o-o 12 Bf4. Now obviously BxB ^ Ne4 etc is the main line, and 13 .. Ng4/Qd6 are known sidelines.

What I find slightly strange is the treatment blacks twelth move deviations. Psahkis & Pedersen only seem to treat 12 .. Nh5?!, 12 .. Nxd4?! and 12 .. Qc7?! which don't make sense. (or fail tactically in the case of 12 .. Nxd4). It seems odd to consider 12 .. Nh5 and not 12 .. Ne4/Ng4. Even 12 .. Kh8 with a vague (easily stopped) threat of 13 .. Nxd4 makes some sense since it's a mildly useful move after black plays e5.  It's rather slow though. 12 .. Ne4 just doesn't seem to work that well.

12 .. Ng4 however seems a fairly reasonable move since 13 BxB QxB 14 Ng3 e5 is known from 12 .. BxB 13 NxB Qd6 14 Ne2 (Qd2 is recommended here) Ng4 etc and meant to be fine for black. Similairly 13 g3 BxB 14 NxB is a known line (from 13 .. Ng4) and meant to be ok.

13 Qd2 is the move white wants to have played if black plays 13 .. BxB but it's not clear black has to: 13 .. e5!? 14 PxP NxP 15 NxN NxN seems reasonable. There are some rather annoying Nf3+ allowing Qg5+ x f4 ideas around. Still got dark squared bishops around.

I have a feeling that white's best might be 13 Bg3!? . Now swapping on g3 besides not making much logical sense also suffers from various Bxh7+ allowing Qxg4 etc ideas. Also development isn't so easy for black. So 13 .. e5 seems sensible and then eg 14 de Ngxe5 15 NxN BxN etc might be a way for white to try and get the usual kind of pull after a quick e5 break. Maybe the tempo from Bg3 is enough to keep this managable? 

So it's not any kind of wonder move. It is however *definetly* better than 12 .. Nh5 and I would say a lot more logical than the much better known 12 .. BxB 13 NxB Ng4!? (Why swap dark squared bishops first unless you're going for Rxf3 etc? You get a tempo on the f4 bishop anyway.).

I can appreciate that this is less fun than the exchange sac (although that's rather burnt out), it's also not that independent of course. Still it's confusing it's such a rare move and not mentioned anywhere - am I missing something big here?! Even if 13 Bg3 keeps a solid edge it's worth mentioning since it's not an immediately obvious move.