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Ramp and tug operator -push back A B737-800 passenger aircraft operated by a national air carrier was being towed to the taxiway where the tug driver intended to change the way of towing from pushing-back to forward-towing. The aireraft experienced a free backward sliding after the rug was separated from the aireraft and the aircraft failed to stop until it hit another aircraft, owned by the same operator, on the taxiway awaiting to be towed to the ramp. This incident caused the serious damage of the left-hand side elevator and horizontal stabilizer of the said B737-800 aireraft and the structure damage of left-hand side cockpit of another aireraft that was hit. The investigation conducted by CAA revealed that the direct cause of this incident was that the mechanic, who was a member of the towing team, sitting in the cockpit failed to follow the stipulations in the AMM to establish the brake pressure to 3,000 PSI prior to commencing the tow. He also failed to monitor the residual pressure during several successive on-and-off towing activities, thus resulting in inadequate pressure available for braking when it was needed. Moreover, there was no wheel chocks provisioned to stop the aircraft during the tow. Recommendation: present your observations from the casel