Most don't use painkillers; they need to feel if something is wrong... basically, it's training, relaxation, and dilation. Most of the girls who appear in the videos are cam girls who are penetrated almost every day, so they have plenty of practice...
All the videos have cuts and edits (albeit very basic) where, obviously, the girl rests, lubricates, prepares, etc... While it may seem very violent, there's also some acting involved...
I often feel that watching a BLW video is like watching a cam performance, but edited to save us the dialogue and preparation that are normally seen... and the tokens, of course.