Heat was always intended as the mechanic for measuring just how much trouble each PC was in, right from the very beginning of Pond Life, the aborted precursor to The 'Hood. In that game, heat was a waste product of the jobs you did: you gained it as you took actions and having too much penalised you, especially in the end-game scenario of working out your character's ultimate fate. There were a couple of jobs that allowed you to lose heat, the most basic one being 'Go to jail', where you chose to take your PC out of play for a round and have them serve time for their crimes.
When it came to The 'Hood, I knew I wanted heat to be in there, but I wasn't sure how, and it went through a number of changes of mechanics before it settled into its current form:
- As I said previously, for quite a while, you advanced whenever experience+heat equaled 5, but you only reset experience to zero after advancing, not heat. I really wanted there to be a way for your heat to reflect the speed at which you were learning the tricks of surviving and thriving in the 'hood, but this method proved to be too confusing and counter-intuitive. Other methods I toyed with included having heat also act as experience, but then that made it hard to use it as a countdown as well; then there was a special advance list for heat, like the one now used for debt; and also an end-of-session roll+heat to see if you earned an advance that session, replacing the normal experience system.
- Another variation that lasted quite a while was the MC's turn up the heat move, which was much like the harm move from Apocalypse World: when you gained heat, you rolled+heat right away to see if there were any immediate consequences, up to and including getting burned on the spot. This also got modified, to a hard move the MC could make whenever a PC missed, but it was too harsh and unpredictable, as it didn't play fair with the players' expectations and could easily short-change them on their characters' stories.
- The main thing I got wrong with heat was seeing it as too much like harm: I treated it as a big, serious deal that was handed out rarely and had to be avoided at all costs, despite having written two basic moves that were expressly about getting rid of it. It wasn't until my first actual face-to-face playtest that I realised I was doing it backwards and that I should be encouraging PCs to take heat, as doing so was one leg of the three-legged stool of the game's economy, with the other two being payback and debt. In order for that economy to function, the players had to be chasing heat as much as their characters wanted to avoid it.
- The Feelgood's healing move existed before the details about being down and out were finalised: Tommy asked how the Feelgood could heal you from being out if that meant you were already dead, so not only was the move in that playbook modified as a result, but the descriptions of being fine, down and out got a make-over, to clarify that you could take a lot of punishment and still be fine; you're only down when you can't move without help.
- The tighter definition of those states also fed back into some of the moves: it was easier to see the difference between taking the hard way and getting rough once it became clear that you could rough someone up incidentally, without causing them any serious, lasting harm.
- Tommy also suggested the street doctor, a back street surgeon the criminal underclass could call upon for their medical needs, which made sense as a playbook that would have something to do in the 'hood: where there's a wound, there's someone with a needle & thread and a bill.