Your potency - enemy's tenacity = X
X is your chance to land a debuff. Your enemy always has a minimum of 15% chance to resist, even if the above results in 100% or more.
Since Daka only has a 75% chance to stun on the primary hit, you have to succeed on that roll first just to inflict the stun in the first place. Then the potency/tenacity check kicks in. Some examples, assuming you landed the 75% chance to inflict:
Your Daka has 80% potency and the target has 40% tenacity = 40% chance to land a stun.
Your Daka has 110% potency and the target has 5% tenacity = 85% chance to land a stun. You have 105% chance, but the 15% minimum resist chance takes effect here.
Your Daka has 5% potency and target has 110% tenacity = You cannot inflict stun because they end up with over 100% chance to resist.
Tenacity Up is +9999% tenacity and Tenacity Down is -9999% tenacity, making the former effectively debuff immunity (except those which "cannot be resisted") and the latter effectively always 85% chance to land.
Tenacity Down can't be resisted and supersedes Tenacity Up, e.g. if the target has Tenacity Up and you attempt to inflict Tenacity Down, it will land and the target now has only the 15% base resist chance to any subsequent debuffs.