This game uses eye tracking to let you play with your own eyes.
To use brain control you need to have Muse Headband.
Dont worry if you dont, you can still use your eyes to control the game.
Make sure you calibrate your eyes correctly and spend some time training. It may not seem that easy at the beginning, but once you get the idea, it will entertain you like no other game before.
For those using Muse Headband:
You will be asked once to record your meditation baseline. Then you will be able to control the size of the character with your focus.
If your friends want to play, just record their baseline before playing the game.