Double jump cancelled down airs. When characters who have a special kind of double jump like Lucas, Ness, Mewtwo, or Yoshi, where it lasts for a longer than average time can do stuff like this. Basically, doing an aerial near the start of the jump causes the jump to be cancelled, your character doesn't go nearly as far, but you reset your characters momentum, which means that if you do an aerial near the ground, you'll fall straight back down rather than jumping and doing an aerial while moving in any direction (up, to either side that you jumped towards).
This means that you can throw out an aerial without moving around, and stay right on top of the enemy, in the case of spikes like Ness's down air, especially fast ones... like Ness's down air, this means you can repeatedly spike the enemy without really giving much opportunity to escape, and without having to chase the enemy far. It's also useful for covering your characters landing, basically just preventing anyone from going near you while coming back down, although that's more of a Yoshi and Mewtwo thing.
At a higher level of play, it's uses get a lot more complex, being a very useful tool for all these characters...
Specific example from the video? Or elsewhere? Can't test anything right now, but I'm assuming it's either just double jump tech in some cases, and wave bouncing on others, also iirc, there's always a slight stalling property to magnet, so normal b reversals are possible too. Haven't played PM in a while, and also have avoided Ness and Lucas specifically for a long time now.