Internal recording

I once loaded a music file into Audacity. It appeared as a track on the screen.
Then I pressed Record, and the music played, and while the music was playing another track was being created below it, identical to the first one. The music was being recorded on the second track.
But after I pressed Stop, I could not do the “internal recording”(my words) again, though I tried. The music would play, but the track below it was blank. The music was not being recorded.
What went wrong ?
(I was excited to discover that audio playing in Audacity could be recorded by the same Audacity.)

I think this is what you mean: