One of the more intriguing aspects of the NFL off-season is theorizing which team will be selected to appear on HBO's Hard Knocks, a supremely in-depth feature of NFL training camp.

The five-episode series is an absolute thrill to watch, as viewers are given a taste of the intensity during camp and the often incendiary relationships between coaches, players, and front office executives. (Season 13 showcased the Tampa Bay Buccanneers.)

Of course, any of the 32 teams can volunteer to appear. Though given how invasive Hard Knocks tends to get, and how many (potentially negative) headlines the show can generate, teams will usually shy away. Hence, Hard Knocks tends to be considered more of a "mandatory requirement" than an "opportunity".

And, as it turns out, choosing a team for the show is done via a very specific selection process, but certain stipulations can "save" a team from having to appear. Three of those "saves" that can exonerate a team from having to appear: (1) starting the year with a new coach, (2) making the playoffs in the previous two seasons, or (3) appearing on the show over the last 10 seasons.

Factor in those three rules, and there are only six teams "vulnerable" to being forced onto the program.

Here are the six teams who would have to be on Hard Knocks if chosen.

Kahleel Bragg contributed photo research.