An uppercut is bringing your fist up and punching Uke in the chin (or the part of the sphere that I assume is his chin), you just brought your fist down onto the top of his head. I could only describe that as a... downercut?
[19:39] <Birdflu> I'm just sad that I can't give myself one
[19:39] <Birdflu> I'd have a great time