Summary
After connecting with a Launcher and Super Jumping into the air, press the Special Button to perform an Air Combo Finisher.
The Flying Screen
Air Combo Finishers can only occur after a Launcher connects. Otherwise, the Special Button will not ever behave like an Air Combo Finisher. Also, if you connect with an Aerial Exchange, the character coming in can also complete the Air Combo with an Air Combo Finisher.
When you connect with an Air Combo Finisher, you will cause what was referred to in previous games in the Vs. series as the "Flying Screen." When the Flying Screen kicks in, your opponent is knocked down and back to the floor very quickly from the air. You'll also hear a whistling noise whenever an Air Combo Finisher connects. The camera view will pan down and follow the victim to the ground and your character will get scrolled off the screen.
Once the screen stops moving, you'll reappear in the playing field by dropping in from the top of the screen unless the screen scrolls so far sideways that you are scrolled off the side where you will then appear back into the fray from the side of the screen.
The Power of Air Combo Finishers
What makes Air Combo Finishers so potent are a couple of things. First of all, it causes a Hard Knock-Down. Now, in previous games in the Vs. Series, the Flying Screen meant one main thing: limitations. You weren't able to Super Jump again, perform any Special Moves, etc. etc. But this is not the case at all in Marvel Vs. Capcom 3! You have absolutely no limits after an Air Combo Finisher so, when you land the Hard Knock-Down on the opponent, you're free to do anything you want to continue the combo... especially because, not only does it cause a Hard Knock-Down, it causes a particularly long Hard Knock-Down.
When you land an Air Combo Finisher, the opponent will bounce on the ground when they hit the floor. However, this does not count as your typical Ground Bounce. During the entire time they are bouncing, they are considered Floored, which means only OTG moves can hit them. But because they bounce first, before they hit the ground, you have a really long time to land an OTG on them, whether it's one you possess yourself or one from a Cross-Over Assist, and continue your combo. This is great for characters like X-23 who do not have any Hyper Combos they can perform in the air, so they don't have any easy way to land big damage during an Air Combo. So instead, X-23 can end the Air Combo with an Air Combo Finisher and land with a fully charged Ankle Slice, which has the OTG property, to pop the opponent off the ground and cancel that into her Rage Trigger Hyper Combo.
The other benefit of Air Combo Finishers is that, because they are Hard Knock-Downs, they ignore Hit Stun Deterioration (see Hit Stun Deterioration section). In other words, if you manage to launch the opponent into the air particularly late in a combo so that the next hit would pretty much mean the opponent will automatically recover right away thanks to Hit Stun Deterioration, you can connect an Air Combo Finisher instead to drop them to the floor. That means, after you land, you can try to connect a Hyper Combo with OTG property that will connect fully despite Hit Stun Deterioration or you can call in an Assist or activate a power-up for your character or just get yourself in a better position to continue offense on the opponent rather than letting them recover in the air.
Strategy Corner
Sometimes, however, you will have to time your Air Combo properly in order to allow yourself some 
hits. If you perform the Air Combo too quickly and land the Air Combo Finisher too early in your Jump, you'll actually land back into the screen slower and not have enough time to connect with your . X-23, for example, needs a decent amount of time to fully charge up that Ankle Slice. So when you go up for the Air Combo, try to space out your moves properly so you hit with the Air Combo Finisher later in your Jump so you can land quickly with enough time to fully charge up that Ankle Slice. Remember, you do not have to chain moves into each other immediately: you can actually delay each move a bit. So in this case, you would want to do that for X-23 so you can be as close to the ground as possible by the time the Air Combo Finisher connects so you have enough time to charge up for the fully charged Ankle Slice to the opponent.