I remember doing a demo flight with GA's David Wheatland a few years back now. If I recall that aircraft had the "Gate" on the prop lever. I was told the "Gate" was, as Jaba says, for noise abatement.
I couldn't help thinking that if it were my aircraft that "Gate" would be removed, I didn't see any point in restricting the aircrafts performance.
To be fair though as silly as the idea of the gate sounds, it's probably the only way to meet noise level requirements for certification in some jurisdictions. They only worry about the peak noise, not how long it goes for.
I think PAC had to go to a four bladed prop for noise requirements for the 750 XL sold in some parts of Europe. These weren't certification requirements but airfield requirements where there were cumulative daily decibel limits. e.g. you could have 1 take off at 100 decibels or 5 take offs at 20 decibels.