When I started looking for a solution to the collision problem I came across flag enable-hot for the first time. And I was confused. I found hints saying it should be part of the XML file (the last property). But setting it in a XML file made the model completely invisible.
Greping through the C++ code I found FGModelMgr::add_model@130 , where flag enable-hot is used for setting flag SG_NODEMASK_TERRAIN_BIT. But this code it not reached when enable-hot is set in the XML file. Only when it is set from Nasal in addition to the source path of the XML file and before calling put_model(). And only in that case the model is visible but will not cause a collision.
So according to my tests my impression is that there are two different ways enable-hot is working (plus a possible third way in animations). But, well, I might be wrong.