The first sac gives tons of practical chances because there are so many variations and move orders -- sure some are unsound, but I think this is one of those cases where preparation carries the day. It's not so unsound though if the best black can do is draw right? 
I've defended similar sacs as in the 2nd diagram. King goes to g8 and is safe.
orangehonda, i meant that the best white can do is draw, in theory if black plays perfectly, but the point is that these sacrifices are only meant for fast games :D
In the 1st one, you could to sac a Bishop on f7, not a Knight. I've had a few wins with this, and it's very good for for blitz.
ok if bc4 in that diagram then black plays e6!, in order to counteract this
Interesting stuff. The 1st one probably quite playable for blitz, though since black can most likely play moves like ...Ndf6 and if queen checks maybe hide out on f7 I definitely would get the feeling in a game that it's not right.
The second one is interesting too. True black is safe enough on g8, but he still needs to demonstrate how he's going to... well, develop! I suppose black can just slowly unravel, with maybe ...h6 getting rid of the g5 knight or maybe ...Qg6 first, with possible ideas of taking on e6.
Interesting stuff. The 1st one probably quite playable for blitz, though since black can most likely play moves like ...Ndf6 and if queen checks maybe hide out on f7 I definitely would get the feeling in a game that it's not right.
The second one is interesting too. True black is safe enough on g8, but he still needs to demonstrate how he's going to... well, develop! I suppose black can just slowly unravel, with maybe ...h6 getting rid of the g5 knight or maybe ...Qg6 first, with possible ideas of taking on e6.
The first position (or sacrifice anyway) has seen GM play, and some GMs play 5...c6 instead of 5...Nd7 simply to avoid these complications.
@orangehonda: for some reason in both diagrams I thought it was black to move. As often is the case, one tempo changes things a lot!
Isn't there something similar called the djory defence or something like that?
I missed the Hellers game above. Try 10.c5 instead of his choice.
I missed the Hellers game above. Try 10.c5 instead of his choice.
Working through this a little further, White ends up with a small positional advantage. So 8.c4 isn't the bust I initially thought it was.
This is an alekhine opening sacrifice, really good for blitz if you ever run into it :D
The next diagram shows a c3 sicilian system piece sacrifice
Amazing as it is that black is cramped and white gets near equal material by taking the rook via f7 with the knight, my engine fritz declares this as winning -2 for black is winning here!!!