grammar - Structure of clause with even if

We must stop, even if briefly.

I suppose this is equivalent to even if it is/be briefly?

But what does "it" stand for? The act of stopping? But a noun cannot be connected to the adverb "briefly" like that, right? I suppose we could assume this is a passive construction where "done" is deleted, but I don't really think that's correct.

Answer

This sentence

We must stop, even if briefly.

is a subordination by means of the subordinating conjunction 'even if'.

The subordinate clause 'even if briefly' means in this sentence 'even if [we must stop] briefly', where the part in brackets is stripped to avoid repetition and because it can be inferred.

In the non-stripped sentence you can see the verb on which the adverb acts.

We must stop, even if we must stop briefly.