[ORAS] The rustling bush/grass

[Nyatalie]

I just got pokemon oras this morning (GTM +8) after playing for a while, i was trying to make a rustling grass to appear (like in the demo) but i found out it takes longer or does it only happen once per route? It is also said that a pokemon found there has a chance to have its HA and a "rare" move (i found out they're
egg moves). I know this is knew and quite confusing for me, so i wanted to know your thoughts or opinion on it. :D