If I knew that my.z = NULL, perhaps I can run a timer while this is true. Then depending on how much time it was, this will tell me how much damage to inflict on the player. That is why it seems like it might be helpful to know if my.z equals NULL or not.

As far as the my.z value on stairs, I do see a my.z value in DEBUG_VAR() that quickly decrements as the player runs down the stairs. When the player runs off the side ledge of the stairs and falls in mid-air toward the bottom, the my.z value in DEBUG_VAR() disappears completely, until the player lands on the ground, to where I see a much lower my.z value in DEBUG_VAR().