Another detail: which foot you have forward will tend to "draw" either a shomen or a yokkomen. If your leading foot is on the side opposite the strike (ai hanmi), a shomen is more attractive for uke, because it is easier to bypass that lead foot (and hand). But if the leading foot is on the same side as the striking hand (gyaku hanmi), yokkomen will be more attractive, as uke will need to get around the lead. I believe that is why shomen is usually practiced in ai hanmi, and yokkomen in gyaku hanmi. Further, these are examples of intentionally drawing/initiating an attack, as opposed to standing there and waiting for something to respond to.