How does essence affect healing? I read something somewhere that it does but is there a rules reference or explanation ?

Thank you,

Only matters for the Heal spell, IIRC.

It also affects first aid. When applying first aid to someone, you get a –1 penalty for every 2 points of Essence below 6.

Check the Healing Modifiers table, p.253, SR4A

I'd thought there was something to that effect, but I couldn't recall offhand and am AFB.

All forms of healing except natural are made more difficult by lost essence. First Aid and the Heal spell have already been mentioned, and there is also the problem of bio and cyberware being rejected from the body with Regeneration (although Essence lost to other phenomena do not penalize Regeneration except where they reduce the Magic attribute).