DS90UB914A-Q1: Want to understand what BIST is actually doing

Robert,
BIST mode enables the BIST pattern to be sent from serializer to deserializer and it monitors the FPD-Link data stream for errors once the deserializer acquires lock. The PASS pin indicates the status of the error checking. The PASS pin goes high after lock is acquired indicating there are no errors. If there are errors on a frame the PASS will go low for half a PCLK cycle to indicate an error.
You can refer to the BIST timing diagram in the 914A datasheet (Figure 20) for additional details.