High-intensity hacks like SpinBots, Anti-Aims, and Rapid-Fire.
For the purist, it represents the ruination of a classic. For the technologist, it is the last frontier of retro cheat development. For the bored veteran, it is the only place where a 20-year-old game can still surprise them.
In a standard "legit" server, using third-party software to gain an advantage (cheating) is strictly prohibited and results in a ban. In an HVH server, cheating is not only allowed; it is the requirement. Players do not connect to these servers to test their manual aim; they connect to test the capabilities of their configured cheat software.
CS 1.6 is an old engine (GoldSrc). It handles latency poorly. HvH players abuse "ping spikes," "fake lag," and "teleport hacks" to desync from the server. If you aren't where the server thinks you are, you cannot be shot.
If you decide to enter that arena, remember the HvH golden rule: Trust no hitbox. Log your console. And for God's sake, don't use paste.