When you start up
Left Brain, Right Brain, you'll notice that you need to turn the DS sideways. Actually, you'll do a lot of flipping the DS over and over to play the game. The first option you'll see is Ambidexterity Check. It does exactly what it sounds like it would do. First, it asks you which hand is your good hand. Then it takes you through a series of tests to determine how fast you are with that hand. Afterwards, it has you turn the DS around and do the same test with your bad hand. Then, it compares them for you.
The next option available is Exercises. Here, you'll create a profile to track your progress. You can have different profiles for up to four players. When you first create a profile, it has you enter a name and then tell it whether you are right or left handed. Then you'll notice you have several modes to select from. The first one is Left Hand (or Right Hand). Inside here, you have five levels to select from, each with three different tasks in them. Level one is the easiest, level five is the hardest. Once you select a level, if you've never played it before, it will have you play first with your dominant hand to get evaluation data to compare your non-dominant one to. After that, if you want to update your dominant hand data, which you should probably do at least every once in a while, you can select Data from the mode select.
Another option inside Exercises is Balance. Balance essentially does the same thing as the Ambidexterity Check, but it stores your score inside your profile. The last option inside Exercises is L vs. R. In L vs. R, you navigate a maze. The first time you play it, it'll have you run the maze with your dominant hand. The game will record your exact path. When you then play it with your non-dominant hand, you'll see two balls on the screen. The red one will be following the exact pattern that it recorded before so that you can compete hand against hand. It's pretty cool actually. I played it again with my right hand competing against the recorded right hand path and it's weird how I followed almost the exact same path a week later, down to the slight swerves around curves.
Left Brain, Right Brain also provides multiplayer modes. You can play against a friend with only one copy of the game by selecting to share your game.