It remains 'open' when the engine is off. Take your seat off and turn on the key, you will see and hear the valve sweep thru it's arc to 'closed' then 'open' again.
This device has been installed at the factory primarily to meet various Govt noise restrictions. If you look at it in reverse, it has been developed for power, I shall explain.
The legislation is basically speed specific in its wording. If the noise control was handled by fixed or permanent baffles in the exhaust system it would restrict performance at other speeds.
In the past some manufacturers have used excessively high final gearing ratios so the bike would pass the noise test by using very low RPM's. That makes the bike a pain in the proverbial .... to ride in the real world. First gear can be too high for idleing along in traffic, or drag racing/wheelies. Top gear can be too high to use at legal h'way speed.
HD did this. To alter the gearing for toothed belt final drive can get expensive, Moto Guzzi differential replacement was even worse. My Cali would get the same economy in both 4th and 5th gear cruising at 115kph, the 600rpm difference meant that in 4th gear at 3700rpm the motor felt a lot smoother. Also, acceleration was greatly improved because the motor was higher in it's torque curve instead of labouring.
So our little device closes up when it needs to, so your EPA boffins don't have aperplexi. WOFTAM (waste of flamin time and money) coz we throw it all in the bin and fit what our bikes need, NOISE!
The surging you experience is more related to the lean mixture of 14.7:1 air/fuel ratio our bikes are set at with the narrow band O2 sensor. The sensor has a major influence on the way our bikes behave/perform below 4500rpm or less than 25% throttle.
See Possible solution to reduce surging & poor throttle response?