When building the keyString to perform an index scan, we build a Ordering object incapsulating the directions of each column in the index definition. The interface for 
Ordering::make expects a BSONObj where each field is either 1 or -1, i.e. it accepts the BSON holding the index definition. In the SBE byte code we have at disposal an integer whose bits are either 0 or 1 depending on whether the direction is ascending or descending, so we allocate a BSONObjBuilder to create a BSON object that look like an index definition (using empty field names, btw).
This allocates and deallocates a buffer on the heap, resulting in a trace in a profiler run, that we could remove by adding a version of Ordering::make more friendly toward C++ (a vector of boolean?)