Well....I suppose in some ways the last clip could be taken from two points of view

You could look at it, like I was the one training to absorb the strike in some Chi Gong body conditioning way or like the Systema clips I’ve seen where they are absorbing punches, sort of training to absorb the strike with my spleen or stomach muscles not I’m not doing that at all, it’s just hurts less when I breath out and helps absorb the strike on the surface of my body, something which if i get the timing wrong passes the surface muscles and is felt internally and causes a sort of spasm and is very painful .if you look closely when Keith is apologizing it’s when his strike has gone deeper than intended and how My body reacts more to the strike, hence the internal striking

Rather than me, training to absorb strikes with my body, He’s learning and refining a few different ways of finishing a hand strike and testing lightly on me to see and feel the effect they have on me and what it feels like when it has the desired effect, then like with the stabbing hook he’s recreating the same type of impact/effect on the water bag, with the human body being "around 60% water and the brain 79%water " and soft tissue etc, I find water bags are perfect for this. Then I’m holding the focus pad on my hand pad to feel if he’s doing a harder version of what he trained on me and on the water bag.

I find the stabbing impact on the hooking strike Keith is practicing is very similar to this clip of Bas Rutten and his Liver/spleen strike. And because of the mma Gloves it allows him to tuck the thumb in and hit with the inside of his middle knuckle, which gives more driving stabbing power

 There’s three types that I teach,a light snap a deeper snap and a hooking or ripping snap these are the same techniques taught to me by Dennis Jones in the earlier clips.and ive found it can trained on the end of any punch or kick to improve explosiveness precision and ultimately has more chance of achieving the desired effect,again this is just my take I'm sure others may see it differently.
hope this helps you guys