why do displacements count as knockups?

Poppy's ult makes them untargetable yea

And Yas can ult off blastcone if an ally hits an enemy up actually, or he hits it and QSS's the knockup to ult

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Barring Poppy's ult and blast cone (I think), that is.

Poppy ulti had a unique effect that it makes the enemy untargetable for the duration of them being airborne. So Yasuo never has an opening to use it. Knockback+untargetable, knockback ends before they are targetable again. It was designed that way so it didn't make whoever got hit by it feel completely helpless. The point of the ulti isn't to completely nuke/cc chain to death someone, it's just to remove them from current fight. If it didn't have some drawback, too much of her power budget would have to be in the ulti (again) for her to be viable in any other way.

Blast cone I assume you mean Ziggs knocking himself back? 'Friendly' knockbacks don't count for Yasuo, otherwise, shit like rocket jump, or every single time autoattacked, they could be ulti'd by yas...which would be just straight dumb. So they made a special rule for it, that the airborne effect had to come from yasuo, or his team for it to 'count.'

Edit- I'm dumb, was thinking blast cone was the name of Zigg's Satchel Charge. But still same rule, if Yasuo, or someone on Yasuo's team was the one to hit the blast cone plant, then he should be able to ulti off it. But if someone on opposite team of Yasuo triggers it, Yasuo should not be able to ulti, iirc.