I think this position could use a tag, since it's pretty distinctive and isn't easily found with any combination of tags. Rule34.xxx has multiple names for this position, as they are wont to do, but "sword swallowing position" looks to be the most popular. I'll suggest sword-swallowing fellatio or sword-swallowing irrumatio.
It's sometimes tagged (or mistagged?) as reverse fellatio, but that tag's definition specifies that the bottom should be on their back. Maybe that tag could be renamed to something more intuitive?
Other kind of related tags: deepthroat, throat bulge, head back, snap my choker (phrase)