Let's say that there is a transformer with 10 loops on one side (side A) and 100 loops on the other (side B). If ther is a rms 10V potential difference produce by a generator on side A, then there would be a rms 100V potential difference on side B. It would seem like the resistance through the wire on side A is constant. Since rms voltage is constant, the rms current should be constant as well. Therefore, it would seem like the rms power supplied by the generator is constant. However, different resistances on side B would cause different amounts of power to be needed by side B. Because energy is conserved, power supplied by the generator is equal the power wanted by side B. Therefore, the rms power supplied by the generator is not constant. So how does side B "tell" side A how much power it wants?