It's not magic, but if you use the same control inputs to turn upwind or downwind, there will be no difference in airspeed coming out of the turns. (As Post # 53 notes, you may well not use the same inputs, and that (trying for a consistent over-the-ground pattern) is one thing that messes up some people's downwind turns. If you make any turn starting at an airspeed just above stalling, you will stall unless you do something to decrease your angle of attack. This has nothing to do with "turning downwind" (which you seem to be using to mean turning into the downwind direction; this is not what modelers usually mean by "downwind turn"). The earlier post about flying from a balloon and the last post's example of a fly in your car make the point nicely: an airplane flying in a moving mass of air cannot react in any way to that motion except by being carried in the direction of the wind at the wind's velocity, no matter what direction it is flying. There is no wind at all (gusts aside) relative to the plane. You feel the wind on the ground because you are standing still relative to the ground. In the air, nothing makes your motion depend on what you're doing relative to the ground.

It's not just the downwind-turn myth that results from failure to understand that a flying plane cannot react to a steady wind in any way that affects its flying. Some people are convinced that a plane flying a crosswind leg will "weathervane" into the wind without any control input. It's just not possible: what force could possibly cause it to do that?

None of this is controversial.