Neil Adams Judo Guru sells his patented version of Uchikomi bands (resistance bands for drilling Judo standing techniques).
These are really cool for power in your grip, as the resistance bands have judo gi / kimono material sewn into the tops of them, for you to grip onto.
These are also designed for you to get power and precision in your technique.
Can be used to drill all sorts of techniques.
I also know of some Aikidoka who have amazing grip strength and monster forearms
due to doing traditional Subori Bokken repetition practice.
The techniques that make up the core techniques in the Aikido syllabus (in most Aikido styles)
come from Traditional Ju Jutsu schools who created techniques based on kenjutsu movements.
The same Subori done with a Bokken can also be practised by those who practice Kendo.
As Subori, done regularly, will give you not only strong wrist muscles, but also
better control over moving the sword.
For throwing, many Judoka will choose on the contrary, a light grip with the strongest part of their grip often being found in the little finger, this is due to it being closest to the opponents body, and gives the most flexibility in movement with the wrist compared to a rigid closed strong full handed grip.
On the floor however, where both fighters are less mobile and both are less able to generate the power to significantly move their partner, tight full hand grips may be more desirable, due to short term control it gives.
If the above ideas still don't float your boat, and you want something more mystical and romantic.
Perhaps watch some of the old 1970s Jackie Chan Kung Fu Movies.... or young athletic kung fu students learning to carry massive ceramic jars full of water with their cat claw hand techniques....
