Zapco DC Intro - Manual (Page 7)

DC-Series Speaker Output Impedance
For Your Information:
Zapco Rail Voltage Monitoring is a system
to protect your sound when speaker
impedance drops below normal range.
Example: At 4Ω a DC352 has 100W RMS at
rated distortion. At 2Ω it has 175W RMS at
rated distortion.
But, what happens if there is a problem in a passive crossover and there is an impedance drop?
Or if a ported enclosure causes a 2Ω woofer to go down to only 1Ω?
The rail voltage monitoring system reduces rail voltage so the amp does not go into low-impedance
protection. Your music keeps right on playing. If you will be playing below 2Ω often, then you can engage
the Low-Z switch to lock in the lower rail voltage
(Did you see the Low-Z Switch? Do you want to know more?)