This training is to harden your fists. There is better training (Push-ups on fists), but if you cannot do it right now, this is recommanded.
This basic way to harden your fist is to stay in the pose of the left photo. The bottom of your dojang or the place you train should be hard too.
The first thing you focus is that you should support with on two fists for long time.
Long time means more than 3 minutes. Master Lee Kwang Hee recommands 10 minues. But it will be very hard.
The exact point of your fist is also very important.
The two inside bones of your fist should be pressed. It means, you should not press any of outside two bones of your fist.
See carefully the photo of right side.
Additionally you should spread your backspine, to be straight.