♡ 75 ( +1 | -1 ) Do you mean king's knight? If so, I believe it's because of two possible reasons.
1) They haven't committed to castling kingside, so there's potentially not a problem with going all out to break the pin (playing h3 and g4 as white).
2) There are some positions that I can never seem to actually find, where after Nf3 ...Bg4, h3, black plays ...h5! Yes, it sacrifices the bishop. But after hxg4 ...hxg4, black has his rook on an open h-file (b/c black hasn't castled) and will swing his queen over. This only creates mating threats when white has castled. But like I've said, I've only managed to lose a bishop through this -- maybe someone could post the move order that makes this trap work.
♡ 94 ( +1 | -1 ) Ruy Lopez, Krol VariationI knew this sounded familiar to me, though perhaps it's not quite what you'd heard. In any event, the line runs 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Ng4?!
Here the apparent intention is to continue 6. h3 h5 7. hxg4? hxg4 where black has a decided edge given the tactical possibilities involving ...Qh4. Of course, if white plays something less greedy (such as 7. d4), black may find himself wrong-footed. The interesting thing about the Krol Variation, though, is that its major proponent is not some let's-throw-a-monkey-wrench blitz fanatic, but a well-known correspondence player, Poland's Wladyslaw Krol.
There's not a lot of data on the opening...indeed, niether the ChessGames nor ChessLab public databases has a single game with 5...Ng4. ChessBase has 13 games by Krol with the variation, where he is 5+ 5- 3=. So feel free to draw your own conclusions.
♡ 19 ( +1 | -1 ) Ruy Lopez, exchange variationThis is one of the "main lines" in the RL exchange: 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bxc6 dxc6 5.0-0 Bg4 6.h3 h5 White can't take the Bishop immediately - e.g. 7.hxg4? hxg4 8. Nxe5? (Nh2 stops the mate but W is still losing) Qh4 9. f4 g3 and Qh1# mext move.
♡ 50 ( +1 | -1 ) The Lasker rule...... Is mentioned in one of Dan Heisman's Novice Nook articles. Check out the Novice Nook #37 thread. This 'rule' goes: Don't pin your opponent's King's Knight against the Queen before your opponent has castled. I suppose the reasoning is that if the opponent biffs the bishop by advancing the h-pawn 1 square, followed by the g-pawn 2 squares, there is no question of weakening the castled king's position. So, if you wait until the K has castled, then the pin is a bit harder to break by such means. I guess... Cheers, Ion