That's a good idea Romy. That topic definitely deserves more room... I'm not sure whether to answer here, or in the new topic, but since my answer is more about play, I think it'll better stay here.
I have no experience with traumatised horses. My horse is (or better was) very shy and sceptical towards humans in the beginning and it took us a while to be able to play. And I'm still working on his initiative in playing. So here are just random thoughts about playing with scared horses.
I would define playing as an activity where the participants can try things out in a controlled and secure environment, safe from consequences and repercussions that the same thing may have in the 'real world'.
For me that means that I have to create such a safe bubble for my horse in order to stimulate play. I think it's helpful to have such a place marked by either time (e.g. a 'let's play-cue'), or simply by location. A riding arena for example is the perfect place to establish as 'playground'. What that means to me is, that in that special place at least at playing time, errors have to go unpunished. Or better: there are no errors in play
. And of course initiative will be rewarded. Cooperation even more so.
I will only use positive reinforcement in play. Negative reinforcement (pressure and release) and of course P- is severely hindering initiative (see: learned helplessness).
I do however use negative punishment (in that case: removal of myself from the play) if I feel myself uncomfortable with the play. (That should be the right of any playing participant, btw.) That's especially important in wild play.