Complex sentence: Before Dan and Amy go to the drop off, a video on the DeOssie phone shows a gun at Nellie's temple and the trigger twitching, before being abruptly cut off.
Split into simpler sentences: Before Dan and Amy went for the drop, there was a video on the DeOssie phone. In it, there was a gun at Nellie's temple, and the trigger twitched, and the video was cut off.