I was always under the impression that a critical hit, sniper ability aside, did twice the damage the attack would have done otherwise. One final 2x multiplier to the attack. However, I would love for someone to explain what it really is to me because after a battle I was just in it clearly is not the case. I had umbreon out. Was steadily boosting with curse and wishing every couple turns. Had a metagross mean looked and all it could do was waste turns with brick break which by the 4th curse were doing very little dmg (10-15% or so). I was going to wish one more time before I baton passed except this turn when he used brick break the damage kept falling after the expected 40 hp or so. I *sighed* and was like 'lol useless crit.' ...but the after 80 hp it kept falling. I became somewhat tense and didn't really understand what was happening or why. I still couldn't think of any explanation as hp dipped into the red and when umbreon was KO'd I just didn't understand at all. Naturally the little 'critical hit!' message popped up but as far as I knew he shouldn't have been able to do nearly that much dmg even with a lucky crit...which is why I thought I was always safe to curse/wish away as long as I trapped a non-fighting type, mainly physical attacker.
So someone please explain why a critical hit in this case would do around 5x the dmg and not just 2x...it didn't end up losing me the battle but it made thins much more difficult than they needed to be and I still don't really get it.