I always use it with gaurd on, although disturbed by someone else asking me a question I stood up grinder running in right hand,gloves ,goggles and ear protection, annswered their question, went to grab the handle with my left hand and hadnt realised I had rotated the grinder in my right hand, first finger of my left was dragged between the gaurd and disc, it took all my guts not to move just turn of and wait for it to stop all the time it was grinding through...well straight to hospital,because I kept still it had cut through my nail,the flesh and just into the bone, healed ok,but a scar on my knuckle where the gaurd dug in due to blade pulling finger in.
So the rule now is ,grinder on and doesnt matter if a angel(yes female!) walks in starkers I dont move untill that blade stops.