Extreme knitting is knitting taken in non-traditional directions.
It can be extremely big, like Rachael John knitting 1000 strands of yarn with what appear to be tree limbs. If you've not seen this video, take a few minutes to watch.
It can be extremely small, like Althea Crome's beautiful tiny garments.
It can be an extreme technique, like Kory Stamper knitting two socks at once using double knitting.
It can be extreme in materials used in place of yarn, such as wire, fiberglass, and lead.
It can be extreme in what is knit, such as a tank cosy, a motorcycle, or a Ferrari. It might be internal organs. It might be toilet paper (scroll down to see it).
It can be extreme in how knitting is used, like knitting graffiti artists who tag trees, lamp posts, and park benches.
It can be extreme in being combined with other activities. My search turned up people who knit while they drum, use various exercise equipment, and climb mountains.