A simple term to describe why this is wrong?

[TheOriginalSteamingNit]

I know this is wrong, and I can sort of explain why, and I can give ways in which it would be right - but what I want is a phrase like 'sentence fragment' or 'comma splice' or 'dangling participle' to use!

Lots and lots of students write things like this:
by doing this, it allows the writer to create an atmosphere of....
by using metaphor in this way, it means that the text is....

So the problem is basically that the 'it' doesn't really respond to anything, and isn't really anything - and when I see 'by', I expect to see a subject - 'by doing this, Shakespeare achieves...'. Or, just write 'Doing this allows the writer to....'/'this use of metaphor enables....'.

But is there some short-hand term I can use here, anyone?

[JoinYourPlayfellows]

Are they changing the subject of the sentence?

By doing this, the writer creates an atmosphere of...

By using metaphor in this way, she creates a text that is...

Who or what is this it and why has it made its way into a sentence that seemed initially to have a different subject?

[TheOriginalSteamingNit]

Yes, it is a misuse of 'it' to become the subject, perhaps?

[chateauferret]

A noun phrase is being replaced with an adverbial phrase of manner. Correct of course is "Doing this allows one to..." Or "By doing this, one might...". The verb 'allow' requires a noun phrase for a subject which is given by the verbal noun 'doing' (in my example). 'By doing' is an adverbial phrase and can only serve to describe the action of some verb.