Can anyone define exactly what constitute a "jam".
For example, is a very firm grip on a senior yudansha consider a jam? Preventing a technique from working - a jam? Not knowing a technique and unable to flow with the nage - a jam?
And, when is a jam polite and when it is not?
Hope to hear from you.
"Jamming" would be any action which stops the flow of movement. It can be offensive or defensive, in other words uke or nage might "jam" a technique.
It's not what is "polite" it's what makes sense martially. If someone stops your technique but is totally "open" then "jamming" in that way was not martially effective. Ones partner should make one aware of this by hitting him.
"Jamming" is usually used effectively as a "bounce". I "jam" your movement and let your power rebound off my power then I give the movement a new direction.
"Jamming" can also be used to simply crush your opponent's defenses for striking purposes.
Simply stopping someone's technique is not good ukemi. If you grab someone strongly so they can't do a technique but they are still able to strike you or kick you, then it was bad ukemi. So if that's what is meant by "jamming" than it' not a good idea.