The 100% potency is not strictly accurate but it is a way to help understand what's going on. The starting point before tenacity and potency is that a debuff WILL be applied , as opposed to will always NOT be applied. What this means is that if they have 10% tenacity and you have 0% potency it is 90% likey the debuf WILL be applied on first check, but there is a second check as all debuffs have a 15% chance to be resisted. Many people assume if the tenacity is higher than the potency it will not apply , this is not the case. There is also a second check , and again this is not a minimum 15% tenacity , it is a straight 15% chance to resist any debuffs regardless of the tenacity vs potency check.
To work it out , take the potency as a decimal , add "1" and subtract the decimal tenacity. I.e 50% pot vs 125% ten = (1+ .5) - 1.25 = 25% chance to apply on first check , then 85% chance to apply on second check. The most practical example is if you want to make sure wampa does not get stunned by iden they have to have 100% more tenacity than the idens potency after all bonuses.
Some examples.
100% tenacity vs 100% potency = 100% chance to pass first check 85% chance to pass second check so 85% chance to apply.
0% tenacity vs 100% potency = 85% chance to apply
201% tenacity vs 100% potency= 0% chance to apply
Hope that helps.