In my ever humble opinion, keeping your arms up does two things to a ride:
1) Psychologically, it involves both the "surrendering" of yourself to the motions of the ride, as it was mentioned before in this thread and the acceptance that you are not in control of the ride. By doing this, you give in to the fear that has you gripping the lap bar in the first place: the fear that something will happen to you on the ride, be it fall out or lose control or what not. When you are not holding on, you accept that the only thing keeping you in the seat is the lap bar, and that piece of machinery now controls your fate, and not your own body.
2) Although the effect is minimal, the human flight-or-fright response results in a natural tendency to push yourself down into the safety of the chair. Therefore, when you hold onto the lap bar, you are securely keeping yourself in a position, decreasing the effect of the G-forces (particularly negative-Gs) on your body and lowering the enjoyment of the ride from a strictly scientific and physical point of view.
Besides, it's just so much more fun to keep your arms up! ;-)
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The Luv Monkey has spoken...
NITRO kicks it into high gear...only at Six Flags Great Adventure!
Track Record: 28 and counting...