There's a couple of questions that will be wrapped up in this.

Right of the bat, under the Togashi Tattooed Order school techniques, it lists Earth Fist, but under the kiho section itself it refers to it as Earthen Fist. I'll have to post that in proofreading if it hasn't been done already.

The kiho Earthen Fist allows you to knock a character prone after activating it if they fail a Fitness skill check. As another effect, if you spend a void point while Earthen Fist is active and successfully attack someone suffering the prone condition, then you double your bonus successes against them.

To activate the kiho, you must be in the stance related to the element of the kiho. So;

Question 1: Does it make sense to be able to activate the Earthen Fist kiho, and then change into another stance next turn? Because if that's acceptable, you can change into Fire stance, and take strife as bonus successes. Unfortunately, the prone condition says nothing about reducing TN to hit in that scenario. Even still, the damage potential of that seems pretty high. I'm just not sure if that makes sense, or even if that's what the devs intended.

Question 2: Under the prone condition, it says "Removed When: At the end of their turn, if the character did not perform a Movement action, they remove the Prone condition." (Page 172) Does it seem weird to anyone else that you can't just take a movement action to stand up? Or that there are no penalties for being prone, other than only being able to move one range band?

I understand to an extent what they're saying. If you spend your turn crawling around, you aren't able to take the time to stand up. And maybe they're trying to mitigate an issue that happens in other systems, where you knock someone down and they stand back up before anyone can do anything. But, I'm still not quite getting it.

1: I can't find a reference to having to stay in the stance to maintain a kiho, so I believe you can switch stances unless someone can find something else. It lists the things that end a kiho, and a stance change isn't one of them.

2: I think you're hitting it exactly on the prone. I believe they are trying to simulate the fact that it takes TIME to stand up. Games rarely simulate this. So when you get knocked down, you can either not move, in which case you stand at the end of your turn (instead of moving, you spent the whole turn standing up). Or you can spend the turn crawling around instead of trying to get to your feet. There will probably be some kind of kip up technique at some time that gets around this. It's also a great way to spend opportunity if you're in water stance.