So promos will be a match of the next rank. They are to test if you can hold your muster in the new rank.
Riot will give you three matches (five for a Full Rank change) where your team will have at least one troll or afker and or the enemy team will have a Plat+ Smurf or Duo Smurf + Boosted Friend. If you can win this Shitfest then Riot will reward you with the promotion then give you 3-5 games of the same where you will lose all of them and fall back down to the Rank you just came from.
You will do this 8-10 times until you luck out and the Troll/AFKer Riot put on your team actually plays and or the Smurfs on the enemy team are to stones/Drunk and feed because (this is my smurf I dont are. It will keep my MMR low). Then you will get to the new rank and you learn from the past 8-10 tried to no play this game again.