The phrase 在瞬间 zài shùnjiān seems literally to mean 'in an instant'. But usage-wise, is it more like 'at some point'? Or am I off the mark?
I think the better way to analyse "在瞬间" is to break it down to "在"+"瞬间" or even "在"+"瞬“+"间".
"瞬“ originally means "[the eye] opens and closes, or, blinks". Because our eyes blink very quickly, this character extends to another meaning-"very short while, very short time period".
"间" means "the interval between two things[or time points]"
So "瞬间” literally means "the interval between the time points when the eyes opens and closes", OK, just means "very very short time period".
"在" is a preposition here and means "in", so "在瞬间“ means "in a very short while", "in an instant".You could also say,
Also, "瞬间” could be used as an adverb to mean "instantly, immediately"(=在瞬间 in this case).
I have never heard it used to mean "at some point." As far as I know, it always means "instantly" and is used to describe something which happens in the space of a brief moment. One illustrative phrase is "瞬间即逝" (shùnjiānjíshì), "to disappear (in the blink of an eye)." Indeed, 瞬 itself is quite close to the meaning of the English phrase "the blink of an eye" and 瞬间 preserves that sense.
I don't think 在瞬间 is used as often as 一瞬间， 瞬间，在那一瞬间，etc, which means at that very moment.
However, I can think of one situation where 在瞬间 can be perfectly used.
This sentence means the bridge collapsed in no time. So yes, 在瞬间 means in an instant in this case.
If you can provide more context, I'll be more than happy to follow up.