One relaxing exercise is to swing the arms left and right while standing on the spot. Look at your mid-section and while twisting from left to right you will probably notice a 'hiccup' in the middle of the movement. While your arms swing once, your mid-section swings-stops-swings. This start-stop-start movement can be transferred through your arms to uke - with interesting effect - when performing say katate-dori techniques. Twist the mid-section slightly, stop, and twist again and uke's grip is lessend - ready for technique.
Note: If your mid-section does not react as stated above, watch others - at least half a class of people typically do this naturally. It can be learned.