Smoke grenades in CS 1.6 were volumetric particles. A legitimate player is blind in smoke. However, many OpenGL wallhacks rendered player models outside the smoke layer. A cheater could see bright green models running through the gray cloud, resulting in "smoke headshots" that looked impossible to a spectator.

Developers typically use tools like to find specific OpenGL function addresses and "hook" them to inject their own logic. A typical hook might look like this: