Air carrier technician reported while working on a switch and accomplishing test of the B777-200 Emergency Power Assist System; the slide fell out but did not inflate.

Narrative

I was assigned to Aircraft X and was assisting/ continuing with work the dayshift Tech's A and B were working. They were replacing the door open switch which was found to have failed during the Door Emergency Power Assist System (EPAS) Battery test Log #XXXXXXX. The test was first accomplished on shift the night before. The slide assy was still in place and safety pins installed at this time. While working on the switch; a tool was dropped behind the slide. At this time; we removed the slide retrieved the tool and reinstalled the slide removed safety pins and installed slide cover on log #YYYYYYY with no issues with supervisor; Tech; myself; and inspector A present. Transitioning to do the door EPAS test I was told by Tech A that the door EPAS switch needed to be in the armed position to get the green light on the EPAS battery Aircraft Maintenance Manual (AMM) XX-XX-XX-XXX-XXX-XXX. At this point because of my position relative to the door handle; I believe Tech A began to operate the handle to the open position; then I took over and continued to open handle to obtain the amber light on the EPAS battery for the test. What we failed to do before moving the door handle was to put the EPAS safety switch back to the disarmed position which was on the next steps of the AMM before operating the door handle; so the door cylinder deployed; and the slide fell out onto the entry stand but did not inflate. Being that I did not review the AMM prior to starting the test; I wasn't aware the EPAS safety switch needed to be moved back to the disarmed position to obtain the amber light resulting in the incident.