"This helps us how?" vs. "How does this help us?"

Question

In the sci-fi movie Inception by Christopher Nolan, in the first level of dreaming, they kidnap Cilian Murphy and Tom Hardy tries to get some information from him, by impersonating Browning, his uncle.

Then they sedate him in a car. Tom Hardy removes the bag from his head, gets down from the car, and says to Di Caprio's character:

That boy's relationship with his father is even worse than we imagined.

To which Joseph Gordon Levitt's character replies,

This helps us how?

Is this a correct sentence? Why didn't he say

How does this help us?

Answer 1

It is very informal, and ironic, but correct as such. Properly punctuated, it would be like this:

This helps us...how?

It suggests that the speaker is trying to come up with a reason why it helps us, but can't think of a reason (because there is none), so he ends the sentence with "...how?", an anacoluthon. That's why it expresses strong doubt: the speaker expects that it does not help us at all. A related but different construction expressing the same kind of informal and ironic doubt:

This helps us because...(?)

The question mark is optional.

Answer 2

By constructing the sentence in this way, the speaker strongly suggests that it doesn’t help.

Answer 3

"This helps us how?" is completely valid. It can either be used as an emphatic alternative to the question "How does this help us?". It gives the effect that the questioner is either hesitant to agree or to make the person who suggested something look stupid, pretty much. It can also be used so as to verify what the person said or just have it reiterated That's only my (unsubstantiated) opinion. Hope this helps.