From Mod Wiki
string key -- Key to look up the effect from. integer joint -- Index of the joint to play the effect on. boolean loop -- Whether the effect should be once only, or loop.
- If the joint is invalid, the effect will be played at the entity's origin.
- The key must start with 'fx_'.
Looks up an effect on the entity and plays it at the specified joint, and returns a handle to the effect.