The trainer scripts have a few "myths" that must be cleared up in order to fully understand this modification:
First of all, no script is activated when the trainer spots you. That is the true reason why trying to place a normal script in that location (instead of a TrainerBattle starting one) would crash the game. The game, in fact, loads the second byte to find the type, the two following bytes for the trainer number, and so on, as activating a TrainerBattle command would normally do, even if the first bit is different. As the game error checking for the Trainer battles have been programmed to return badly if the trainer doesn't exist, the game crashes because it loads an invalid command parameter.
Second, the Trainer flag is not really a flag. In fact, the game checks for only two possibilities in that byte: if it's 1 or if it's 3. 1 it's the basic trainer activation, and the behavior we are used to see in-game. 3 is the reverse of 1, in the sense that it only detects you if you go behind the Overworld, and not facing it.
Third, the characteristic !, followed by walking to you, is not a in-game response, but rather one special, special 0x37. It usually does nothing, as when you can activate it in a script, it hasn't got a person number, and when you have a person number, it's because you talked to that person, and it doesn't need to walk towards you. But the trainer routine deals with that, using a routine to set the last checked OW as the OW using the script.
Foruth, the trainer flag is checked not only when you are in front of them, but rather whenever the sprite is loaded into memory, but only runs when several conditions are met. Besides the obvious check for you in a clear path from them, it also checks for your pokémon number, and if you have at least one, it checks if you have at least one pokémon able to battle. If all conditions match, it tries to load the script data.
But, if it accesses a script, there must be a way to make it run it. As such, this hack came to be. Basically, what was done was add one extra option, where it checks if the Person event last two bytes as a flag number. If 0, it's a normal trainer sprite, but if it's greater than that, then it's a flag, and it will check if it's active. If not, runs the script asked. If it's set, it ignores the person script. The real reason there is a flag here now is to prevent staying in a loop, where you would be constantly "harassed" by the OW. The flag is automatically set as soon as the script is placed to run, so you don't need to place a setflag in the start of every script. Also, as an added bonus, the game will also perform automatically run a small "pre-script" that has the specials needed, making it look just like a regular trainer event.
