Looking through Reddit, this question arose. With a couple of conflicting answers.
It's when there's a modulation into the V key - but that actually puts it fair and square back at I.
It's something like an F chord with a G bass (a sort of slash chord).
Neither seems particularly plausible. And either unnecessary. Any better explanations?
It sounds like Reddit's confusion is a confusion in labeling.
"IV/V" indicates what is often called an "extended tonicization." In a basic tonicization, a secondary dominant chord (like V/V) briefly tonicizes a new pitch (like V). But in an extended tonicization, you can move to a new key area with more than just a local tonic, like a IV/V moving to a V/V and then to V, creating a IV–V–I motion in the key of V. But as you say, this particular example isn't all that convincing, since IV/V is just I. But something like ii6/V would be a great use of this concept, since that would be an ultra-rare vi6 in the original key.
But a IV chord with scale-degree 5 in the bass is best notated as IV/5, indicating a slash chord with scale-degree 5 in the bass. This can be understood as a V11 and is sometimes referred to as the "rock dominant," which I discuss in Is there a specific name for the use of IV chord over the V in the bass, e.g. F/G in the key of C? (Note that the question and several answers in that post also confuse the notation!)
If it's notated specifically as F/G it's a 'slash bass', an F triad over a G bass note. Sometimes confusingly labelled 'G11' or even 'G9sus', so it's lucky the much clearer 'F/G' is available!
But, using Roman numerals, IV/V means 'IV of V'. Not as common as the 'secondary dominant' V/V, but quite possible if we want to view the dominant key (and some surrounding chords) as a new but temporary tonic.
Not sure what you mean by 'puts it fair and square back at I'. Yes, 'IV of V' is the same chord as 'I'. Just as 'V of V' is the same chord as 'II'. That's OK. A pivot chord will have two names, one in the old key, one in the new.
