when you press down B, ZSS does a jump flip with cool special effects. This flip can be used for recovery as well as mobility, as it is completely lagless. it acts as a third jump, and has invincibility on frames 3-12. Also, while flipping, you have great control over your aerial mobility. Use all this to your advantage. This is an amazing mobility tool. And can be used for recovering and getting out of bad situations. Also, if you hit a wall while you are in this move, you do an automatic wall jump for extra recovery. However, if you press an attack button while you are using this move, ZSS performs a downward kick that kills very early, but has lots of ending lag. This is a very viable kill option for ZSS, but you must not be predictable with the move, otherwise it becomes very punishable. The tip of the kick spikes the opponent, which is even more flashy
. A very big problem with new ZSS players is that they use this move too much and/or they use the kick a lot when they shouldn't. When using it as a movement tool, don't use the kick! The weak hit of this move is when you land on the opponent with this move, but don't press the attack button. ZSS head stomps the opponent. This will bury opponent in the ground or softly spike aerial opponents. Remember that the head stomp has lots of lag anyway, and ZSS will perform the head stomp on an opponent even if they air dodge or shield the move. This can get you punished or killed, so be careful. If you bury an opponent, feel free to punish in any way, such as Up B on the spot, charge an Usmash, Dsmash or jump-fast fall until they break out, then Uair chains into Up B if they are at the right percent. Something very important to remember about this move is that not only can you aim the kick left or right by hitting that direction on the joystick when you press the attack button, but you can do the same with the move itself. If you hit forward right after you hit down b, you will flip forwards instead of backwards. Also, because the move grants extra aerial mobility, you can hold towards the stage when recovering with it to jump even farther. When recovering with it, remember that you cannot hit up b until the move is completely over. If you try for another recovery option to early, ZSS will perform the kick, and you will fall to your death in lag. However, if you do actually connect the kick on your opponent in midair, you will jump off of them, negating the lag.
This is from my guide if you want to check it out:
http://smashboards.com/guides/full-guide-to-everything-zero-suit-samus.602/
Good luck!