In trying to solve my "Gearbox Fault No Drive" problem on my 2005 DB9, I have looked at the emergency parking brake release mechanism, that is located under the driver's side rear passenger seat. The manual explains that there is a cable activated lever that should be used when the car is going to be towed. Once the lever is pulled, the cable pulls on another lever on the side of the transmission, which releases the 'Park' position of the automatic box, that is engaged whenever the ignition is switched off. Thus the car is free to 'free-wheel' and be pulled up onto a recovery platform.
Even though the manual has a couple of pictures showing the location, I was still confused by the elaborate layout of the mechanism and didn't realize that a bracket on the top of the device has to be removed before the cable can be activated. So I thought I would take a couple of pictures to show what you will encounter once you remove the seat squab and aluminium seat tray (10 bolts!)



Here I am holding the top bracket that has to be removed with the two screws mentioned in the manual. The looping cable is actually just a tether... so you don't lose it(?)

With the bracket out of the way, you can now pull on the looped cable, or yellow lever and ratchet it into the extended position, thus pulling the pawl out of the transmission. The car should now be free to tow or push.

Once you are done with the maneuvering, you can press this sprung black button to release the ratchet and allow the pawl to re-engage in the transmission. The car is now back in 'Park.' Reattach the upper bracket and reassemble the seat tray. Done.