A Line Mechanic; after forgetting to set the hand brake; describes a chain of events that contributed to his de-icing truck rolling into the fuselage nose section of a company B737-800 aircraft.

Narrative

After being instructed on a new; four truck deice procedure at a conference room with all involved in the deicing [procedure] (about 30); we were released and assigned to deice a B737-800 at Gate C-11. It was already pushed back and waiting. I had my truck in position off the right-hand nose with Mechanic Y in the pod above. Mechanic Y asked me on the intercom why Mechanic Z was plugged in with a headset where we were to start [deicing] on the radome. I gestured to Mechanic Z; but I believe he misinterpreted it. I asked Mechanic Y if he'd like me to get out and ask. I'd been in neutral with my foot on the brake waiting to start; but another truck was having truck issues. I hopped out forgetting that I never had pulled the hand brake. I crossed in front of the truck; over the tow bar and grabbed Mechanic Y's arm. He turned around and yelled when he saw the truck moving. I turned around and saw that it had hit the plane. It was very obvious to all around and inside the plane.I [previously] got my rig into our position; put it in neutral; and was waiting the word to start [de-icing] from the primary one truck. One of the trucks on the left-hand side reported truck issues with the pump I believe; so we sat for a while Equipment Ground Support was called and next step was determined. The process was interrupted. I always set the hand brake if I ever hold in one spot long. I even set the brake while the wing is shot if it's a big job as in the ice. Everything was moving fast; as the plane was already pushed when we were assigned. Not that unusual in ice operations. I forgot that I had not set the brake.I ran up to the truck [after the truck hit the airplane]; opened the door and confirmed it was in neutral and the hand brake not set. I got in and another Technician in front; hand signaled me to back up. I backed it up and the plane 'settled.' Someone told me to see Supervisor X; who took me to his office; told me to have a coffee; relax and come back. He did investigation paperwork and took me for my drug/alcohol test. After that I was released to work the schedule; not deice.Like many accidents; things were out of the norm. A new [four truck] procedure; late notification; events not as trained (Technician on headset at nose); deice truck broke down; holding in position longer than normal unplanned and simply forgetting to check the brake before getting out.