I was just going to say......check Nick's video.....
I was confused about stems myself until I watched the relevant videos.

Only thing I can add is watch them several times. I like Nick's choice of music but I usually turn off the sound. We see more when we eliminate the extraneous.
When we watch another person doing a physical action mirror neurons in our brains fire. These are the same neurons that would fire if we were doing the action. It's sort of like the other guy is practicing for us. (Some key details require additional work which is why air guitarists don't pass the audition).

: This video might help explain the process.

:
: http://www.guillemot-kayaks.com/guillemot/blog/nick/shaping_internal_stems