Adam!   a B/F  is forcing air into your Paint cup by blocking the AB nozzle!   Some seem to believe it helps mixing the thinners and paints while in the cup!       It might do it?     but the risks of Bubbling out and making a mess is very present!

Seen on a lot of Japanese Modeler's YouTube videos!    And not a good practice in my book!

Don't want to insult anybody but I find it lazy!    Nothing beats mixing paint in a small container then pour it in the AB!

To each his own!  I guess?