decelerating energy

Hi all,

I was wondering, if you decelerate a large load (gantry), the stepper
motor will deliver the energy back to the drive, in my case a
gecko201. The drive tries to put the energy in the power supply, the
PDF-file on power supply from geckodrive advise that you place a
zener diode, so that the energy can't go back in the power supply
(right?). Otherwise this would charge the capacitor to high levels.
So were does the energy go?, in can't go to the power supply and the
drive tries to put in there.
Thanks.

Hugo

PS this will only happens when all three steppers decelerate.