Actually... not quite true. You can sorta heave to with just the main, at least according to the Pardee's. My wife just finished reading a storm tactics book by them and the two of us have been arguing the merits of heaving to under main alone. According to the Pardee's, you can only do it in medium level air, and I can see why.
The concept is to create a balance between being in irons and heading up. First thing is to sheet your main in tight. You sail an upwind course, and then gradually pinch until you JUST go into irons. You lash your tiller or wheel at this angle. After a little bit, your boat will fall off a little, create a little forward motion, allow water to pass over the rudder and due to the rudder angle, push you into irons again. Rinse and repeat
I don't like the idea since if the wind goes up a little bit, you have the opportunity to complete the tack and could potentially crash jibe if you aren't paying attention. Anyway, the point being, you can sorta heave to under main alone.
Personally, I think you should always heave to using your genoa. It's a tried and true method and far more controllable. As long as you have enough out for it to catch air you should be able to heave to (if your boat will allow you to heave to at all).