When you're better than the people you're matched against, you rank up. When you're worse than the people you're matched against, you rank down.
If you're on average better than a diamond player, then having diamond players in your games tends to increase your winrate, as there are 5 spots on the opposing team that can be diamond and only 4 spots on your team.
If you're on average worse than a diamond player, then having diamond players in your games tends to decrease your winrate, as there are 5 spots on the opposing team that can be diamond and only 4 spots on your team.
If you're on average better than a gold player, then having gold players in your games tends to increase your winrate, as there are 5 spots on the opposing team that can be gold and only 4 spots on your team.
If you're on average worse than a gold player, then having gold players in your games tends to decrease your winrate, as there are 5 spots on the opposing team that can be gold and only 4 spots on your team.
If you're on average better than a silver player, then having silver players in your games tends to increase your winrate, as there are 5 spots on the opposing team that can be silver and only 4 spots on your team.
If you're on average worse than a silver player, then having silver players in your games tends to decrease your winrate, as there are 5 spots on the opposing team that can be silver and only 4 spots on your team.
If you're on average better than a feeder, then having feeders in your games tends to increase your winrate, as there are 5 spots on the opposing team that can be feeders and only 4 spots on your team.
If you're on average worse than a feeder, then having feeders in your games tends to decrease your winrate, as there are 5 spots on the opposing team that can be feeders and only 4 spots on your team.