Some people were asking about the wall grinding mechanic in FMN.. Here's what it's all about:
How to grind: Think of the controls as being like a 4-directional joystick which is always pointed in one of the four directions. A quick swipe or slight movement of your finger is enough to change its direction. Wherever it's pointed is the direction you'll move, unless a wall is blocking the way.
If you're moving along next to a wall, you can set your next direction ahead of time. Point the "joystick" towards the wall to grind against it (you'll keep moving in the direction you're already facing), until you either reach an opening or point the joystick in a different direction.
You can tell you're grinding because some sparks will fly out where you're touching the wall, and you'll hear a grindy sound.
What does it do? Grinding builds up a charge. When you're charged up enough you'll start flashing slightly, and the sound will change. This means you're all cool and powerful and can squish enemies by running into them.
If you keep building up charge you'll eventually start flashing red (or blue if you're player 2). That means you're approaching maximum charge and you'd better stop grinding soon or you'll explode! During this state your speed will increase.
The more you grind the more you'll get used to knowing how far you can push it. Playing with sound on also helps a lot... you can tell by the pitch of the grindy sounds how much charge you're carrying.
When is it good to grind? Pretty much all the time! It's not essential but makes things more difficult and exciting (speed boost), while allowing for much higher scores (squish combo). If you can balance your grinding so as to always be charged up without ever exploding you'll be a Forget-Me-Not champion. Like Kepa. Yay!