When I went clipless, recently, one of the drills I practiced was emergency unclipping. I would ride slowly around the estate where I live and stop on an expansion joint. At first I made sure I was unclipped well before I stopped. Each time I did this I unclipped a little later. I got to the point where if I felt myself starting to go over my natural reaction was to twist my heal and unclip instead of trying to pull up. A couple of time I have been caught and have been able to unclip just in time.