The Opera is a much
hyped Half-Life MOD which is loosely comparable to
Action Half-Life. The idea is that it places
style over brute force, and rather than rewarding linear
frags, you get a certain amount of cash depending on how you kill someone - knifing them will get you more cash than strafing them with an
uzi.
One of the much-hyped features of the game is the dives and rolls, but in practice these are nearly useless because if you attempt one in-game you're very likely to die. They suck, and Action Half-Life had them long before anyway.