The rear arch effect on starships is necessary to allow for the difference between the holding grip position and the relative angle of the forearm in relation to the forward holding grip, which requires the wrist brace extension to be placed slightly lower than the holding grip to ensure a linear holding stance of the starship slingshot relative to the aiming eye (in my case the right hand side eye.
The absence of the arch would result in the wrist and the forearm not being aligned i.e. the wrist of the holding hand would need be angled to the right to allow for the wrist brace to make secure contact with the forearm, with the resulting wrist torsion this implies: not ideal.
The arch effect has no bearing on the actual projectile velocity (at least I don't see why it would).
Hopefully this makes sense... :what: