From the pilot's notes:
''Pressing a DECK TAKE-OFF button on the cockpit port wall releases the locking solenoid and allows the UP button to be pressed in, but does not make the hydraulic selection. The latter is made automatically when the weight is removed from the undercarriage legs''
You would usually do this after the aircraft had been tensioned on the cat.
Sorry. Only just read syncro single's reply. Old age...