My older Model 750's 12V trigger turns the amp on as soon as the trigger is enabled and shuts the amp off as soon as the trigger is disabled. The 755, however, appears to behave differently. From page 8 of the manual:
Make certain that the connection to the processor, or other controlling device is correct, as described on page ?. Whenever the controlling device is turned on, the amplifier will automatically turn on after a short pause. This pause is intentional, and it protects your speakers from damage while the amplifier stabilizes. You may also hear a relay click as during start up. This is also normal.
To turn off your amplifier, simply turn off the unit that feeds the amplifier it’s audio signals. The amplifier will automatically go into a stand by mode in a few moments.