Are Fast TMs Truly Random?
I've tried 7 fast TM on Tryanitar and it always flips between Bite and Iron Tail. If it were a random, I make it less than 1 in a 100 chance of that happening. So, I tried some others that have a 50:50 chance of getting what I want (IOW, 3 moves) and there did seem to be a bias towards certain (I would say "crap", but that's subjective!) moves.
So, I'm wondering if it's not random but biased against certain moves? I could find anyone else that's measured it, however.
Answers
All TMs work such that they change the move to any of the other possible moves for that slot, but they'll never give you the same move you already have in that slot.
In the case of fast moves, each Pokemon has 2 possible fast moves - so a Fast TM will always give you the other possible one.
In your case, it seems you've confused Tyranitar's move pool a bit. You can't get Smack Down on Tyranitar any longer; it was only available as a fast move for a limited time. You can only get Bite or Iron Tail now.
Well, when you have two moves, and you rule out the one you have, the choice isn't random at all, it's 100% certain. You have Bite on your TTar, TM it, and the only possibility is Iron Tail.
However, if you're addressing the randomness of using a CTM, where you normally have 3 possibilities, then the question is interesting. For Ttar there's three possibilities, Crunch (C), Fire Blast (F), and Stone Edge (S). If you have S, and want C, you have a 50% chance of getting it with a single TM. Otherwise, you end up with F, and have to TM again, where you have a 50% chance of going back to S and a 50% chance of getting C. If you go back to S, you're clearly right where you started except you used two CTMs.
Now, some people theorized that it's possible that theres a hidden variable, like on this day, S is the preferred move followed by F followed by C. If that were the case, then when C is on the bottom, you're more likely to get S and F than C. There are reports of people using 5, 7, 12 CTMs and failing to get the move they want, giving up, and then trying again the next day and getting the desired move first time. If you believe the hidden variable theories, best practice is to use 1 CTM per day until you get what you want. Or, you could believe that these reports are an observational effect - the people who get the desired move the first time (likely half of the people who try the CTM) don't post anything, but the ones who fail to get the desired move over and over do post, so readers of the board only hear about people who try over and over without getting the desired move.
Personally, I'm in the latter camp, but since it costs nothing but time to do 1 CTM per day, that's how I do it, just in case there is some hidden variable.
Mew is the only pokemon with more than two available fast moves. Using a fast TM on Mew randomly selects one of those moves other than its current move.
All other pokemon have only two available fast moves. If a pokemon has a legacy fast move, a fast TM will first randomly to one of the two available fast moves, then subsequent fast TMs will switch to the other.
Another odd caveat: You can only use a fast TM if the pokemon has more than one available fast moves. As a result of this, Zapdos caught with Thunder Shock can not be changed to Charge Beam.
