An active terminator, like a passive one, absorbs the signals that reach the ends of a SCSI bus. The passive terminator is designed with the assumption that the TERMPWR line is exactly 5v. Because of cable lengths, multiple devices adding or draining TERMPWR, that voltage isn't quite accurate.

This becomes more a problem as the SCSI bandwith increases and the unabsorbed reflections introduce errors on the bus. Instead of just resistors, it includes transistors that help keep the impedance matched by adapting to the TERMPWR voltage variations.

When SCSI reaches tens of megabits/s, the SCSI signals behave more like radio signals than electrical impulses ... Active terminators are a MUST!