It makes "tickling my chest" into an ... apostolic clause (???).
For those who may be interested, the term is 'appositive', which I assume it's derived from the same root word as 'position', rather than 'positive'. I think of it's meaning as 'in the adjacent position'.
CW, are you having difficulty reconciling my posts concerning the sample sentences from you and Switch that we are currently discussing? I'm not entirely certain about them myself, but I'll attempt to explain what I see as their similarities and differences.
Both are certainly 'appositive noun phrases', i.e. phrases positioned after a noun providing extra information about that noun.
Both are 'non-restrictive', i.e. they could be cut from the sentences without the identity of the noun becoming less clear, or in other words, they describe the noun rather than help identify it.
Both phrases are set off with the 'gerund' form of a verb, i.e. the -ing form of a verb NOT preceded by some form of the be-verb, which is required in all standard progressive tenses (aka continuous tenses) of verbs.
Gerunds are like the Thai silk of the grammar world: if you look at them one way they appear to be one colour (verbs), but look at them another way and they like something else (nouns).
With these two sample phrases, tickling his chest and spouting off like a New York cabbie, the gerund is functioning as a verb within the phrases - but the entire phrases are functioning as noun phrases.
The rule of grammar (for formal writing) is that non-restrictive appositives must be enclosed in commas (or similar punctuation marks), but restrictive appositives should not.
But … I hate the tickling-phrase with a comma preceding it almost as much as I hate the spouting-phrase without commas at both ends??? I don't recall ever seeing anything in any guide which suggests they may be treated differently, i.e. both must be enclosed in punctuation marks to show they are non-essential detours from the main body of the sentence.
I'm confident everything I've said above is correct … but what follows is pure speculation.
I see the differences between the two phrases as consequences of their different positions within the sentences. [Note. I think 'sentences' could be replaced 'clauses', and the same conclusions would follow within clauses of a 'complex sentence', i.e. a sentence with multiple clauses joined by conjunctions and/or (non-)serial commas.]
The tickling-phrase appears at the end of the sentence (clause?). There seems to be no need to alert readers that you're about to take them on a detour away from the main body of the sentence: readers don't need to remember anything to continue parsing the main body of the sentence after the detour is over. There is no reason to interrupt the flow of the sentence with a comma – for that clause in that position.
In contrast, the spouting-phrase appears near the start of the sentence – and it is separating the subject of the sentence from its verb! In that position it does seem essential to make readers aware when a detour from the main body of the sentence is starting and ending, so will know what's going on when they need to pick up where they left off.
I'm pretty sure that not every detour following the subject of a sentence needs to signposted with commas – just because it happens to be non-restrictive – but I think this detour, in this position, NEEDS THEM!
The reason I think this detour needs signposts while others may not is that it misdirects readers' expectations. When a sentence begins with a noun or pronoun, a subject, readers expect its verb to come next, or at least without any undue delay. If there's a comma after the subject readers are warned there will be a detour before the appearance of the verb. If there's some other part of speech after the subject (and I am speculating here) readers are warned there will be extra information about the subject before the appearance of the verb. The major problem with this phrase is 'spouting' looks like a verb, and readers will attempt to interpret it as verb, but it's actually functioning as noun.
Seriously, my brain rebelled when I read "You spouting off …" for the first time. It screamed, "That does not compute! The only way this can make sense if with some form of the be-verb, either are, were or will be, between the words 'You' and 'spouting'. I think a comma after 'You' is needed to prevent readers mistakenly attempting to interpret 'spouting' as the verb in the main body of the sentence (and once the first comma is used a second one at the end of the phrase becomes mandatory).