irst of all you can reach 100% slow resist by having phase rush (75%) and boots of swiftness (25%). Yep, they stack additively.
Second when you get to 135% slow resist (with the rune unflinching) you will be able to increase your movement speed if someone slows you. For some reason it doesn't increase when you have 110-120-125% slow resist.
Update:
It's not about reaching 135% slow resist. It's just about using the first passive of the rune Unflinching when you have 100% slow resist "You gain 15% slow resist and tenacity for 10 seconds whenever you cast a summoner spell.". All the other slow resists stack until 100%. Only this 15% surpasses the limit making you have 115% slow resist, video.
Taric has Boots of Swiftness and the rune Unflinching. He also proc-ed Phase Rush.
PS: Credit to Tylobic (Taric) who came with the idea and asked me to test it with him :)
PSS: Also works with self slows (ex: blitzcrank/tahm)
PSSS: In the video you can see the movement speed of Taric in the box in the left corner (for those who didn't see it :3)
