From reading here and on EnerSys's website, basically, you drain it with a load to 10V then immediately recharge using a 6A+ charger, never exceeding 15V charging current, until it reads something like 14.2-14.4 volts. The high amp chargers are needed as the AGM is able to absorb a charge much faster than standard, flooded lead-acid batteries - and - because the rapid charge absorption can 'knock the stuffing' out of any sulphated areas, possibly re-activating them.
The low voltage is not damaging if only for a short duration. It is the long-term low-voltage periods that can cause irreversible/non=-recoverable sulphation on the plates.
Or something like that.
Yesterday (one page back) at 12:53 PM, the "official" re-conditioning instructions from EnerSys are posted. Here are two short YouTube vids.