As you scan the sims observe K, OUTPUT currents. You should observe at K=0 there is no coupling. At low values of K the output picks up quickly. At high values of K the signal drops off again. You can not see the whole picture here. In the next series of sims I will show that changing K also changes the coils inductance. Loading the coil shift it inductance.
The output is half as much at K=.5 as it is at K=.25. Why is this?