To elaborate on the method of gripping the shinai I talked about in my last blog entry, I extract some figures from the manuscript from Toshio Matsumoto (9th Dan Hanshi), the same document which I quoted for describing the Natural Stance Kamae.
The position of the end of hand grip should rest in the left hand as shown here:
The two muscles circled in the upper figure should be used to squeeze the end of shinai. (Aki Suzuki Sensei form Tokyo once told me the same.) Likewise, the same muscles of the right hand have also the same function.
The feeling is like supporting an egg with these two muscles without breaking it, as shown above.
The right hand should grip the shinai in a way that is hard outside but soft/flexible inside. To explain, he use the analogy of a "bull crap under the sun (hi na ta gu so)", which is hard outside but soft inside (rather interesting don't you think ;)?).
The position and angle of the thumb should be straight and relaxed, directed roughly parallel to the floor, as shown below:
This is connected to what I described with the right hand grip in my last entry.
Hope the above information helps!