While at first this trick might seem fairly impressive, it can actually be broken down to a very simple numbers game in which you are made to feel as if you are in control of the situation, but in reality the possibilities for you are narrowed to only one result.
When he has you move four spaces, and then an additional three, the only two possibilities as far as where you will land are the numbers 2 and 4. The reason for this is because you have started on an odd numbered space, and he is making you move an odd number of times. The result will always be an even number.
When you choose a number, you will land on 1,3, or 5 if it's odd, and 2 or 4 again if it's even. This move is to make the user feel as if he or she is in control of the situation. By this point, the "mind reader" will have no idea which space you've landed on. Even if you choose to move 0 spaces, the trick will still work as you are still on an even-numbered space.
Moving another two times isn't necessary for the trick to work. In fact, every even-numbered move you find throughout this trick (except for the user-choice move and the final move) is completely superfluous, and contributes nothing to the result of the trick itself.
When he tells you to move that number that you chose before, this is going to put you back on 2 or 4 no matter what number you chose. If you chose an odd number, and you have to move an odd number of spaces, the result will be even. If you're on an even space, and you choose an even number, you will still land on 2 or 4.
The boy/girl thing is a gimmick. Another unnecessary move making the user once again feel as if there might be a different result if they were a member of the opposite sex. Trust me, I went through all the necessary operations, and the results of the trick didn't change.
By this point, you will still be on 2 or 4. When he tells you to move two spaces to the right, you are being forced to the number 4 spot.
What's interesting is that there are a lot of moves that the user will make that will not affect the result in the end. If we completely break down the trick, you will still get the number 4 space by doing the following:
Start from 1
Move 3 (or any other odd number) times
Pick a number, and move that many times
Do it again
Move 2 spaces to the right. If you're on 4 already, this will only put you back on the number 4 spot.