How to use the bair
ROB's back air is, by far, the move that sets his aerial game apart from other characters.
It autocancels, it fastfalls easily (just hold down, yo), and it auto-spaces. It can also aut-cancel. These things combined make it a great move.
If you are using it facing away, it autospaces you outside of the range of ANY attack save for fast projectiles and the like. Regardless, it autocancels so you can shield or roll.
You can also use it towards a shielding opponent, fast fall, and then d-smash or d-tilt depending on the location of your opponent.
If you hit someone at low % with a back air that pushes you towards them, you can almost always get off a sick combo. Back-air, d-tilt, down-smash, followed by a u-air and a fall-away laser. If your laser is charged, this leads you to an edgeguarding opportunity.
The back-air is also great for recovering, but we all know that by now. Use it to push you towards the stage.
Something else that's great about the back-air is the delay it has! You can use this to your advantage. If you treat the backair like, say, Fox's back-air, you'll just get in trouble. When you are playing an opponent that routinely air dodges (hellooooooooo Snake), jump into their "zone" and use your back-air. Often they will air dodge quickly, meaning that the delay in your back-air will be just long enough to destroy them.
This also hold true on the ledge! Most people stay on the ledge for all of their invincibility frames, so you can run off and back-air near the end and hit them into the stage for a surprise spike!
Did I also mention that throwing down a gyro then jumping over it and repeatedly back-airing is completely impassable by any character? If they don't have a projectile, they literally can't safely approach you. It's a fun game to play against noobs.
One other trick for those reading this far... did you know if you use your up+b, you still save your double jump? It gives you more horizontal distance to just tap up+b when on the stage than short hopping off, and you move a lot faster. This REALLY helps destory people like the space animals or Snake who like to recover from just out of normal short hop range. Just tap up+b and throw those fairs out, and then you still have your double-jump AND the rest of your up+b fuel! Be careful though; if you use your up+b with improper timing, you'll waste your jump. I generally just tap up+b once and use my jump next so I don't lose the extra height.