Hirate kamae = flat hand fighting position. It could actually be called "slightly cupped hand fighting postion" This is the ubiquitous "post" position that your hands are in at the start of kanshiwa kata. Ok - here's the correction I've finally internalized.
The hand is slightly cupped. Basic right? Absolutely - but over the years my hand had flattened a bit as I was training to keep my elbows in and to wacky explosive things. I just wasn't paying attention to my hands. So now I am again.
Here's a hand:

Obviously not in anything close to the desired postition.
here's a hand that (without the tiny laptop) is actually pretty close to the desired position:

The important thing that I wasn't focusing on is the ridge that is just under where your fingers join your palm. I was unconsciously pushing that ridge forward which was busting the alignment of my hand.
Of course this is tied to my recent fandango with jar training. If the head of the jar you're grabbind is the correct size - then all you need to do is let go of the jars and bring your hands up in front of you in the same position and voila - hirate kamae!
happy cupping,
Dana