A lot of the people I come across seem to be mistaken and think that you can't QSS out of knock-ups. This is correct but also, partly in-correct. To those of you that don't know, there are two parts to a knock-up: The displacement and the stun. Displacement is unique in the fact that it isn't coded as a CC or a debuff so you can't QSS it. Displacements are what Yasuo's ult triggers off of; not knock-ups, and it's one, single, main trait other than displacing you is that it stops you from moving. The second part of the knock-up, the stun, is there to act like a silence, so you can't use skills while getting knocked-up. You can't QSS the displacement, but you can QSS the stun. This is allows you to use your skills/summoner spells and also auto-attack; you can do everything apart from move. For example, if I was playing as Vayne and I were to get Malphite ulted, I can QSS right when the knock-up connects then immediately tumble away or flash away, keeping me away from danger. This is something that puts mobile ADCs above immobile ADCs since mobile ADCs can QSS then dash away from knock-ups while an immobile ADC would have to waste flash.
All forms of displacements are actually coded the same way as knock-ups too. That means Lee Sin Ult, Tristana ult etc etc.