A faller fell a 5 foot diameter tree on steep 80% side slope. The log rolled down the hill and came to rest on top of a fir stump with the upper portion of the log elevated. The Faller assessed the entire log and taped prime bucking lengths along with trimming off the broken end of the upper piece. The Faller proceeded back to make his final cut assessing that both logs would fall away from his work area. When the cut dropped, the lower portion of the log took off down hill and the upper portion of the log pivoted uphill striking him and pinning his legs to the ground. To extricate the Faller, the log had to be jacked up. Worker was heli-medivacced to hospital for examination but suffered no injuries. Worker returned to work the following Monday.