What does Roy's down+B move do?

It's kinda hard to test it - maybe in training or 2P mode, but I think it either dodges the attack like a sidestep or it deflects it back like Peach's move where she holds out Toad or Mario's/Dr. Mario's move where they swing their cape!

When someone shoots something at you he blocks it.

It's a kind of a counter attack if somebody hurts you while you are standing in down+B mode you slash or kick back try it in training mode.

It deflects a projectile at your opponent or just away from you. It depends on the timing.

No it's for if someone with a sword attacks you (like Link) it will be as if you had a perfect timing hitting someone and you both attacked at the same time it's a stale attack. I reccommend you not use it if your a beginner it's all about timing.

It powers up his attack

He can fling homing attacks out of the way and lets say Mario punched him use down+b and it reflects it like backfiring punches. Cool huh.

Roy's Down+B move has many uses. Mainly, when you use it, it is used as a 'counter' attack. Roy's counter differs from that of Marths, as it will deal a lot more damage (depending on the original strength of the attack), but it is harder to pull off.

It can also be used to deflect projectiles, just like Marth's counter. About the only thing it can't be used for is when someone tries to grab you. It's good for about anything but that.

It's defence against your opponents attacks. If they attack you while you are in this position then you will strike them instead.

It's a counter attack/ reflect