I've been to Victor Navone's blog many times, but somehow I've missed this page, which he recently added to and posted about.
Thumbnailing ideas is a skill I'd like to develop further myself. I think video reference is good, but I feel that thumbnails can be a great way to caricature and build better composed, graphic poses. Drawing small is a big advantage, I think because it makes you think more about the pose and less about the drawing itself, but I'm still figuring that out ;)
Navone's drawing are quick and expressive, so I wanted to post and share them in case you've missed them too! I know I've benefited just by seeing how other people sketch their thumbnails.