Consider a two level system - say a single spin-1/2 electron in a magnetic field. To make transition between the spin-split levels the usual advice is to apply an alternating transverse magnetic field at a frequency equal to the level splitting.
But what I don't understand is why the transverse field needs to be alternating at all. If I apply a static transverse field, I will see transitions to the other spin state as well. Why/how are these transitions different than the alternating field case? Why is one way 'better' than the other?