Solo in Arvo Pärt's Symphony No. 3

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I had the pleasure of playing Pärt #3 several years ago. It really got me into his music.

The piece has a nice trombone solo towards the end:


In this recording, the trombonist (or conductor?) messes with the time. The unaccompanied measures are taken twice as fast as the accompanied ones.


What's going on here? Is there an interesting bit of Estonian performance practice to be learned? Does Neeme Järvi just dislike long tones?