In Counter-Strike 1.6 (GoldSrc engine), a is a form of ESP (Extra Sensory Perception) that renders player models through solid geometry. The "F1" designation refers to a commonly bound hotkey used to toggle the hack on/off via a menu or console command.
Modern (and even legacy) anti-cheats like , Cheating-Death , or VAC1 detect F1 wallhacks via:
Wallhacks typically work by manipulating the game's memory or by intercepting and altering the rendering process. This allows the cheater to see through surfaces that normally obscure line of sight.