http://www.jblproservice.com/pdf/Vin.../JBL-SG520.pdf
see pg 20. Power relay (energizer(s)), and unusual test/null (speakers) relay... If the
"balance" were off, you'd hear a signal while the test button was pushed. I assume you
can then tweak the balance control on the pre to maximize the null. Then release the test
button.
Sounds like a convenience really (power relay part), and (important!) only for use with
the intended equipment. The null thing sounds like an unnecessary set of relays in the
signal path, as well as an odd wiring requirement that would confuse most.
Neat thing to have with an SG520. But, for me, it would be to play with, then put away.