As defined by the 2019 NFL Rulebook: Rule 3, Section 40, “Tripping is the use of the leg or foot to obstruct any opponent (including a runner).” Rule 12, Section 1, Article 8 makes tripping a prohibited act.
It turns out that, after a punt or a kickoff that was fielded by fair catch, the team receiving the kick actually has two options: they can run a play from scrimmage (also known as “doing normal football things”) or they can attempt a type of free kick called a “fair catch kick.” If the kick goes through the uprights,