I think there is sometimes a misunderstanding about relaxing.
Telling someone to relax is often a waste of time. People tense up for a reason. Whenever someone is grabbed hard (uke or nage) they will automatically tense up that area as a natural response to resist movement.
When someone grabs you they are doing it for a reason (i.e. to pin, strike or throw you). The only way to 'relax' is to blend completely with their movement. i.e. if they are pushing your arm down, go down with it (keeping your back straight), and then when they have reached their point of over extension, rise up again and do kokyu nage.
If you just focus on the bit that is being grabbed you tend to get hit by other areas. If they are grabbing you with two hands and just holding you this is just plain stupid as you can strike them with the other hand (best not to hit friends that hard - just poke them in the floating ribs or the nerve underneath the ear - gently!).