I was just curious. IMprov has always been an enigma to me. I mean I understand jazz improv to a certain extent, but there is a definite pattern and outline there. This free improv I'm sort of puzzled about. Do you mean you don't think one iota before beginning and it's like you're just listening to the gods
or your "innerself"? You never know where you're headed? I guess I should just break down and try it myself. Give me a hint. 
This answer applies to me and is not necessarily valid for other improvisers.
Yes, no and sometimes. The things I plan beforehand are usually of necessity general matters, vague enough not to inhibit dynamic flow, which must always be given complete freedom. For instance, in that thread where we all improvised on the Wilson/McPartland motif, I realised that certain general things were going to occur before I started. I was going to use two styles in more or less alternating short episodes, one blue, the other romantic. I was also going to put it through the twelve keys, juxtaposed but not harmonically related in the usual sense. Because the tune was initially in F minor I knew that I was going to start accompanying it with the other hand in D minor - for the blues combinations. How I would round it off I hadn't a clue before starting.
For myself, I don't believe in "trying out" stuff before an improvisation, and I didn't in that case. But I do remember thinking about it while in the garden.
But no, you're right, that was exceptional in that in most cases I just sit down and start with nothing. Usually around ten minutes into it, a feedback loop establishes itself, producing a torrent of ideas over which the conscious "I" in the usual sense has no control and becomes an interested spectator. One of my personal weak points is that I like neither endings nor beginnings. Endings still cause me a great deal of discomfort, if only because the state of extreme pleasure must be brought to a close. When I used tape I just played until it ran out, but with this Zoom thing nothing runs out and I have to invent an ending or hope that the phone rings. It's a damn nuisance. Beginnings aren't so bad aside from the fact that most of mine are weak.
The big difference about learning to improvise - and it is a lifetime process - is that you begin with a state of freedom and gradually, over time, work toward order. This learning process is so greatly at variance with the serial, step by step, teaching conventions, not just in music but in everything else, that people, especially highly trained adults find it almost impossible. Most children, of course, are as yet uncontaminated by wearisome serial disciplines and inhibitions, and have no difficulty at all.
I think many prior discussions exist here, involving m1469, quantum, Bernhard, Derek, Wolfi and several others, about specific ways to instigate the habit of improvisation ( there is no single "right" way or easy "method" applicable to all individuals I'm afraid) so have a read through them.