ZSS Down Air Stall?

[g0dzilla]

Hi, I'm Godzilla.
I'm currently making ZSS one of my mains.
I was watching Xanadu 100 : Mars (ZSS) vs Gunblade (Dark Pit, Dx3)
https://www.youtube.com/watch?v=WdmkP5PmxDE
At 0:40 in the video, I see Mars down air and it slows down, like when you lose momentum after getting launched into the air.
He stalls in the air with a slow falling Down Air and then after the down air is over, he proceeds to do a faster version of the down air.
If anyone can tell me exactly how did he do this?

 

[David Viran]

If you use dair too quickly after hit stun then it will do this.

 

[Im Fragbait]

you can also jump then dair to trigger it. i use it as a mix up loads of times