I often see spells say 20% chance to miss or 50% chance to miss like Bestow Curse for example. How does that work? Does a gym just flip a coin and if he's wrong then he does nothing and if he's right he gets to act or?
I tend to have the player / GM roll any dice. If they roll high (eg 4 to 6 on a d6, 11 to 20 on a d20) they get the result they want. Low = bad. This is consistent and easy to remember (since you generally want to roll high in this game).
If trying to hit an enemy with 20% concealment, roll a d20: 1-4 = failure, 5-20 = success.
Concealment Miss Chance
Concealment gives the subject of a successful attack a 20% chance that the attacker missed because of the concealment. Make the attack normally—if the attacker hits, the defender must make a miss chance d% roll to avoid being struck. Multiple concealment conditions do not stack.
Make your attack as usual. If you hit, the defender rolls % dice. If the roll is <= the miss chance, it's a miss.
I often see spells say 20% chance to miss or 50% chance to miss like Bestow Curse for example. How does that work? Does a gym just flip a coin and if he's wrong then he does nothing and if he's right he gets to act or?
When making an attack and your attack roll is successful, roll a d100 (or 2 d10s - one for the 10s digit and one for the 1s digit). You roll 01 to 50 your hit becomes a miss. 51 to 00 (100) you hit. OR you can flip a coin. Either way.
Normally you'll see the type of miss the ability or skill grants. You can roll miss chance anyway you want, as long as it's accurate.
For combat miss chances due to concealmelemt, you roll normally the attack dice. If the attack HITS (it's a success) you now roll the miss chance.
Normally you'll either roll:
For most accurate roll: 2D10 (different colored or roll twice, one for tens, other for units)
For percentages that are multiples of ten: 1D10. Multiply number rolled by 10%
For percentages that are multiples of 5: 1D20. Multiply number rolled by 5%
Most people have the attacker roll. But in truth, as SlimGauge pointed out, it is the defender that is supposed to roll.