When referring to the parameters of :GetPartsInPart()'s documentation, BruteForceAllSlow is listed as a property of OverlapParams; however, this property is not documented on OverlapParams, as its own page, or within script editor. Instead, it is referenced as a property of OverlapParams without any elaboration as to its functionality.
If I had to guess, it’s probably something they use internally. Internals usually never get documented, and even if they do it’s never detailed. Cool find though.
Odd that it would be included in something like that though. Usually those things are either entirely undocumented or have their own page with no other references to them.