The actual consensus rule regarding what is a valid block is determined only by mining-diversity.
By contrast the mining-turnover setting is a suggestion to well-behaved nodes as to how to behave.
If mining-turnover=1.0 then all nodes attempt to create a block when they can. There is nothing wrong with this, but it can lead to forks that then need to be resolved (automatically).
If mining-turnover=0.0 then, once enough nodes have mined to create a valid group for rotation, the other nodes "sit it out" quietly, unless they see that too much time has passed for the next block, i.e. that one of the existing miners in rotation appears to have stopped. At that point they can jump in to fill up the group again. This minimizes the probability of a fork, but it is not an actual consensus rule, and isn't suitable if mining nodes are coming and going all the time.
Other values between 0.0 and 1.0 exhibit a range of behaviors in between, in terms of how likely a node is to behave one way or the other.