The e6 - Nc6 setup is perhaps the most complicated system in all of bughouse. There are a lot of variations to learn and I hope to cover the most important ones in this text.
This is the most common position you can get in this setup. Here, White has 2 main options: 7. Bd2 and 7. 0-0 . I will cover both of them in this blog post. Let’s start with 7. Bd2:
After 10. Rg1, Black's best move is 10...P@d2+:
Remember the P@d2+ move as it's highly critical. If 1. Kxd2 Bxc3+ 2. bxc3 Qxf2+ 3. Qe2, you have to notice that B+N mate:
However, if Black cannot get B+N, this line is fine for White.
The last option is 11. Ke2, which is a very common reply to 10...P@d2+. For Black, I recommend the line I already showed and also encourage Black to follow this line up with 13...N@f3 at the end:
White pretty much has to sacrifice the queen back as their king is very dangerously placed on e3 and has no way to retreat. (Do note that White could also play 13. Kg3 instead of 13. Ke3, but then 13...B@f3 will force White to sacrifice his or her queen again.) After that, the position favors Black since White's king is so exposed on f3. White can always take the g7 pawn with Rxg7, but Black's king is pretty safe as it can run to d8; furthermore, after Bd7, Black has a lot more space for running if needed.
Anyway, it is impossible to cover everything in the above lines, so I just covered the main options only. Now, let's analyze the second option for White instead of Bd2 - castling:
Here, White sacrifices a pawn for a good initiative. Usually, Black follows up with P@e4, but after Ng5, White's attack is more dangerous. There is also the possibility of an early N@h5 drop for White:
After 3. Bxg8 B@e4 4. Nxg7+ Kf8, both sides have good chances to mate although I don't recommend this line to people without supreme tactical vision and very good bullet skills.
The next option for Black here is to play solidly. Instead of 8...Nxc3, Black could just play 8...B@g6, 8...B@h5, or even 8...P@f5:
Or, if there is a big flow of pieces coming, Black could try to be be highly aggressive and play 8...P@g4:
Now Black could play 10...P@f3:
Instead of 11. gxf3, White might play 11. Nxe4:
In the above line, if Black fails to get enough material to mate White, Black is just lost.
Another key idea to keep in mind is that after 11. Nxe4, 11...N@e2+ would be very unpleasant:
A relatively uncommon line that you might encounter starts with 7. Bxe4:
Position after 10. B@a4+ B@c6 11. N@h5:
White can get some initiative going, but still, there is plenty of counterplay for Black after 11...Bxa4 12. Nxg7+ Kf8 or 12...Kd7. If no knight is coming, I would say that both sides have decent chances to mate but White lost a lot of material for it.
Another uncommon line is this line:
At first look, it doesn't look that bad for White, but bughouse is a game with two boards. Let's take stock: White lost 2 pawns and 3 minor pieces (8 Ps) for 1 pawn, 1 minor piece, and a queen (7 Ps). Not only that, but remember that in bughouse, quantity generally beats quality. Therefore, with that many pieces lost to gain the queen, White's partner will just get killed while White has next to no winning chances. For this reason, you will rarely encounter this line.
Yet another uncommon line is this line:
You won't see this in high level games for many reasons. First, 5...P@a3 or 5...N@a4 are pretty unpleasant for White. 5...B@e7 is also a very solid move for Black. If no pieces have been traded on the other board, 5...Rg8 could be played and after 6. e5 d6 7. exf6 gxf6, Black has a solid position in compensation for the sacrificed material.
Let's continue with the most common lines now:
This is also a very common line. If 8. N@h5, Black responds with 8...N@f5. If, on the other hand, White plays 8. N@h5 and Black only has a bishop in hand (to reply to 8. N@h5 with 8...B@f8), Black must use the sit button because after 9. P@h6 gxh6, if White has another piece to drop on f6 or could play P@g7, Black would be in serious trouble. black are in serious trouble.
Now, I will cover the 7...Nxf2 options for Black.
This is the best White could do. For inexperienced players, this final position might look scary for White since there are a lot tactical approaches here, but in all cases White is doing fine. The main problem for Black is that he or she mostly relies on drops to create an attack; unfortunately, sometimes drops aren't coming! 10...P@h3 could be played if Black has a knight in hand, but 11. B@h1 counters it. 10...P@f2+ is also quite all right for White. Let's consider yet another move for Black: 10...P@e4; then White would play 11. Nxd4 Bxd4 12. Qg4. After 12. Qg4, White's threats are much more dangerous; Black's kingside is totally exposed and N@h5, P@h6, and N@f6 are all coming with dire consequences. If Black has no pieces in hand, Black must play 10...Nf5 and after 11. Bxf5 exf5, White's position is better.
Do note that there is another move order that one can use to sacrifice on f2:
Instead of 11. Qe2, 11. N@f3 is even better, but if a knight is not available, this position is also perfectly fine for White. Indeed, this position obviously favors White since if Black wants to attack, he or she needs a lot of pieces to drop. One possible approach for Black is to play 11...N@g4, threatening 12...Nxh2+ and 13. N@g3+. Other moves Black could try include P@e3/N@e3, P@e4, and Bc5; however, with accurate play, there is still no danger for White in all of these lines. Truly, it is not a good idea at all to put yourself in a situation where you highly depend on drops. If you face a strong team, they will punish you with good flow control.
Finally, another option for White is an early a3:
Be careful of this trick as it leads to an obviously lost position for Black. Instead, just go Qe7 and play a calm position:
There's nothing to worry about here. 8..N@e4 or 8...B@b2 are good moves; if empty-handed, 8...Nh6 and 9...0-0 are also fine. Don't be shy to use the sit button often if your opponent sits a lot. 9. P@d6 is the only real threat White has and it's best ignored:
This final position totally favors Black. Even if White plays 11. N@d6+, Black could just reply with 11...Kf8 . White is going to need a hand full of pieces to get in and even then they might die first.
Do note that White can easily avoid all of those scenarios by taking exd5, heading into an Exchange French position.
Finally, the last critical option for White is the early e5 push:
Well, I am really lucky that my friend, MiniGreat, who is also a very strong bughouse player, covered this line very deeply in his blog; had he not done so, this post would have been twice as long! You can read his analyses here.