From QWiki

Server side setting that disables broken ankle behavior added to QuakeWorld by the Kombat Teams mod. It thus allows you to carry on bunny-hopping even after falling from heights causing a "hurt sound" rather than a "thud".

Fallbunny is turned off by default, but recently it's been getting more acceptance.

Trivia: "fallbunny" is actually what the original Quake mod acts like, the broken ankle was only first introduced with Kombat Teams.