Strange I must say...
I guess it's really type specific what exactly the condition levers/speeds/prop levers do. On the 340 going into beta range is protected by the flight idle stop below which you must lift guards on each power lever. When below flight idle on the PL, prop RPM is governed by the bottoming governor that maintains a constant RPM of 1040 to reduce noise levels. When going into reverse, prop RPM is increased to 1200 automatically by the bottoming governor. In beta range (guarded), the PL controlls blade angle directly I believe and the DECU forks in as much fuel as is required to maintain the prop RPM at 1040 or 1200 depending on PL position.
Do you control reverse power with the power levers or with the condition levers on the B1900 respectively Chyenne III? This might be a clue as to why this procedure is used on the Beech and not on the Chyenne.
/LnS