when I sit down to meditate, afterwards i slowly go into corpse pose and then lay their for a few minutes. Currently I use a homeade chinesse meditation bench and so the first thing I do is slide my feet out from underneath me and a little to the right, Kind of looks like a spinal twist at that point. well the other night I woke up in that position. after my meditation I only got to the point of sliding my legs out to the right and bam, off to sleep. Two hours later I woke up in that pose, I guess that is what some people mean by just melting into the pose. 
I had such experience once when I was a kid … I had a very physical day, and I have fallen to bed as a tree cut down, slept all night and woke up in the same position. Never again I managed to repeat this feat … 
I know there have been days where I’ve fallen asleep with full makeup on and slept so soundly on my back that the makeup was perfect when I woke up.