When does the NUnit runner use --explore

We are using the "NUnit runner" build step with the option "Reduce test failure feedback time".

According to the documentation (https://www.jetbrains.com/help/teamcity/getting-started-with-nunit.html), For failing tests, the NUnit is first started with the --explore argument and then the actual test run starts.

We have observed that even when the previous build is not failing, the --explore argument is still used. Which in our case adds a few minutes to the test run.
Note that we have some expensive test data sources.

Our question is, how does TeamCity decide when it will use the --explore argument?
If there ever was a failing test on this build chain? Or is it related to branches?

Thanks
Thomas