Mr. L2 has it right. I'm a Test Pilot but more importantly I've spent a fair amount of time in the mountains on the "seismic trail". What this instructor was probably talking about is making a lz recon while keeping the aircraft somewhere close to the bucket speed (thats how I do them). Lets say it's 45 KIAS. While traveling into a 20 knot wind, the power and attitude required will be the same regardless of the wind (the disk don't know) however your groundspeed will be much lower. As you turn downwind the IAS will drop rapidly (this is readily observable and in fact is a technique for determining wind direction and velocity). The reason it will drop is because the mass/inertia of a system (the aircraft) is not a function of aerodynamics. If you were to maintain the exact power/attitude the aircraft would eventually work its way back up towards the original IAS (a slight increase in power would be required to accomplish the acceleration) but the important aspect in this discussion is that it would take a considerable amount of time, which if you are at relatively low altitude in the mountains, you don't have. This is why you want to make that power/attitude change as you begin the downwind turn, that way you can keep the KIAS up close to that reference airspeed, and keep the aircraft from dropping out from under you. This is why I like to use
bucket speed, it gives you the most excess power available for the maneuvering necessary for low altitude/high DA operations.