Re: Jamming a technique
As a rule of thumb, anyone in my dojo can jam a shodan and above anytime they want. Keeps the ego in check. Frustrating for shodan yes, but you're there to learn. Not to throw people around. And for shodan, you're assumed to be a knowledgeable and dedicated beginner. One who knows he has much to learn and too little ability just yet.
Appropriate resistance is also vital for helping lower kyu to understand. If sensei jams you, there's really nothing you can do about it. Because his understanding of aiki is much more than yours. Same goes for you vs a beginner. Thus, provide resistance, but provide a way out. If they can't feel the way out, show it to them until they do and understand.
But David i think is talking about jamming between peers. And those peers who like to use strength to accomplish their goal. Thus David's jamming of those brute strength waza is more of self defence really. No one likes to be clothesline at the neck in what nage assumes to be correct iriminage or sayonage.