Great links there, Numberless-- I hadn't heard any of those before. Most people just call that rubato; Hal Galper calls it furious rubato
. It's a free jazz thing that started with Ornette or Coltrane, I think, and got refined and prettied up by the early ECM guys. It's a major part of my thing, but it's hard for me to find much to say about it. I call it free time because when I was figuring it out nobody around me was doing it, and I had to call it something.