Question about small っ... or double consonants, I forgot

[TheGodFather]

So I was reading this manga online, and on one of the pictures in between chapters there was a character and a text box that said にゅうっ (I saw it as "Nyuu"). But I saw the small っ and I thought those were used for words that have two consonants... or a "heavier" sound if you know what I mean. Like あさって (asatte).

I'm not completely sure if it was a っ or a random line or something else, but if this has reason I'd like to know, thanks.

PS-currently taking Japanese classes, I'm not fluent lol, I don't even know how to read katakana yet.

Also the manga scan was edited for english, but the picture wasn't.

[Dreams]

A っ or ッ at the end of a word indicates a glottal stop, as though the speaker has been cut off.

にゅうっ! = Nyu--!

But yes, normally, when the "small tsu" is used inbetween syllables it "doubles" the consonant following it.

[Hiryuu]

To be more precise, it's more like your hang on to the consonant of the next syllable for a syllable. If it's an an 's', you stretch it, if it's a k or something similar you can't really stretch out, you pause for a beat.

That is also what we see here, at the end of 'Nyu'. However, because there is no next syllable, it's just a stop.