Regard the following sentence:
The Subject of this sentence, clearly, is Bob. Now consider this one:
- Bob, he's a mighty fine guy.
Here Bob has been shunted to the left of the clause. The word Bob is now like an announcement of the topic of the rest of the sentence. This is known as a left dislocation. Some constituent of the sentence has been dislocated to the left of the nucleus of the clause and then a co-referential pronoun, he, has been used to plug that gap. The grammatical Subject of this clause is now the word he.
Left disclocated elements do not always represent Subjects. Consider the following joke:
- Men, you can't live with them and you can't kill them.
Here men is coreferential with the object of the preposition with. Again, it announces the topic of the sentence.
We can also have right dislocations too:
Here the Subject has been dislocated to the right of the clause. Right disclocations tend to affirm the identity of some co-referential element in the sentence, in this case the pronoun he.
The Original Poster's example
- Just Googling it is bad for your brain
- Just Googling, it is bad for your brain
In sentence (1) the grammatical Subject of the sentence is the clause Just googling it. We can show that this is the Subject by turning the sentence into a question:
- Is [just Googling it] bad for your brain?
Here just Googling it has inverted with the auxiliary verb is. Subject-auxiliary inversion is how we mark interrogative matrix clauses in English. This test is very reliable for determining Subjects. One thing to note, at this point, is that the grammatical function of the word it is Direct Object of the verb Googling within the embedded clause.
The Original Poster's student read this title and interpreted it as sentence (2). Sentence (2) involves a left diclocation. Here, the gerund participle clause Just Googling is a dislocated subject, which has been shunted to the left of the clause proper. It represents the topic of the clause. Taking its place is the pronoun it. The Subject of the clause is this word it, not the dislocated element just Googling. The phrases just googling and it are two entirely separate grammatical entities here unlike in sentence (1).
We can show that the word it is the Subject here by doing the same test we did with the first sentence:
- Just Googling, is [it] bad for your brain?
Here we can see that is has inverted with it.
The actual title is as shown in sentence (1). There are several reasons for thinking this. The first is the lack of a comma after Googling. However, comma usage is quite subjective nowadays, and in addition, this phrase is a title or headline, so we might expect the comma to be left out here.
The other clue is the word just. It doesn't seem to be very effective here if this is a left dislocation. It would make the dislocated subject less dramatic, not more so. Presumably, the motivation for using a left dislocation here would be to make the title more snappy. Lastly, after you read the article, it becomes clear that it is arguing that if one just Googles it, instead of trying to remember it, this is bad for ones memory. The killer piece of evidence, however, is that the title of this article, which has been published on several websites, is normally punctuated like this:
- "Just Googling it" is bad for your brain.
This seems to show very clearly that "Just Googling it" should be considered one phrase here.
In answer to the Original Poster's first question, the phrase just Googling (it) is a gerund-participle clause. In sentence (1) it is the Subject of the sentence. In sentence (2) it represents a dislocated subject.
Regarding the second question, we can indeed use left dislocation. It is more common in speech and in informal writing. Titles and headlines, however, are somewhere where we might expect to see them even in more formal genres. Within the body of a text, we would usually expect the dislocated element to be followed by a comma.