For what it's worth, it isn't that difficult if you break down the mechanics of what they're doing. I know full well I can't do it on demand though!
I've deliberately slid my bike and been successful 2 times. Once was a 3rd gear exit from a roundabout onto a dual carriageway in the wet using Racetec K3s on my way to Silverstone (that was the best one) and I held it for maybe 3 seconds. The other one was about 1 second of powersliding in the dry on truly dead tyres but it got too big for me and I backed out.
What these guys do every time, hitting every apex, dragging elbow, dragging shoulder, sliding it and smoking it... they are several cuts above great riding and I wish I could ride like them
I have no idea how they do this, hard work and practice and I think a lot of offs before trying to be able to do this in my opinion. They are truly skilled riders