Well, i guess that means 300 to 200
30 would translate to 3000 ft, which is pretty much everywhere below the transition level and would be done as an altitude instead. Anyway, for high altitude descend without restriction the autoflight system usually will use idle thrust and maintain the speed with pitch. In an airbus one has to actively start the descend, either bei engaging managed or open descend or use vertical speed mode. A boeing will start the descend on its own if it is in VNAV (normal mode in cruise) and a lower altitude is selected, but only once it reaches its calculated top of descend. Normal descend (and climb) modes are managed (managed by the Flight managemend computer) in an airbus or VNAV (managed by the flight management computer) in a boeing, however there are other modes available. Open descend (airbus speak) or Level Change (Boeing) uses idle thrust or climb thrust to descend or climb and adjusts the pitch to keep the planned speed. Vertical speed mode is available on both and holds the selected vertical speed, airspeed becomes secondary and will fluctuate especially in a climb where it can drop below the normal speed envelope. However it is a very usefull mode for ATC mandated rates and for small altitude changes where it is often better to use V/S than open climb/descend.
Flying manually is a nice pace of change, but done differently with the airbus fly by wire system than in other airplanes in some parts. The airbus system tries to be flightpath-stable, if one is in level flight and just reduces thrust the airspeed will bleed off, the fly by wire system will increase pitch to hold the flight path (level flight) until it reaches a lower speed limit. Therefore one has to actively control the flight path with the side stick in manual flight. In a boeing, same as in a C150, if one pulls the power pack in manual flight the nose will drop and the aircraft will try to regain its trim airspeed.