Conversion byte() question

// Now spin both!
  driveArdumoto(MOTOR_A, CW, 255);  // Motor A at max speed.
  driveArdumoto(MOTOR_B, CW, 255);  // Motor B at max speed.

both "MOTOR_A" and "MOTOR_B" look like the are stored in the value of "byte motor"

How is it knowing to spin both at the same time?

Now that I am thinking about it. It does not clear motor just sets

motor=0
motor=1 and runs it and there would be a few nanosecond delay between the starts of motor A and B