Spoiler Alert (You've been warned)
How did a lowly sanitation stormtrooper come to know how to blow up The Starkiller Base? Watch the Undeleted Scene that explains it all.
https://www.youtube.com/watch?v=MhpDzy7RfTU
Sometimes in the interest of time critical scenes end up cut from movies. We at Undeleted Scenes are fixing those movies one plot hole at a time.