Absolute Anime

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.

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.

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.