A question about speaking in tongues

[hvn_nbr_2]

My concept of the idea of speaking in tongues is from the story in Acts of how the apostles addressed large crowds of multiple nationalities; and although the apostles spoke in their own language, all the people in the crowds understood what they said in their own languages.

That seems to me to be the polar opposite of what "holy roller" churches mean when they do speaking in tongues:

Everybody understands in their own language vs. nobody understands a word.

The speaker is speaking coherently and knows what they're saying vs. the speaker doesn't have a clue themselves what they're saying.

Everything is understandable in real language to everyone vs. babbling nonsense streams of syllables from no language at all.

If they're really speaking in tongues, rather than just excitedly babbling, why doesn't everybody understand? I have a vague recollection of someone sometime telling me that there are other stories somewhere in the Bible about a phenomenon more like modern speaking in tongues, but I don't remember it.

Anyone able to enlighten me?