Mas oyama once said that hand-stand training will give you grip strength and a person have powerful grip with speed can kill a person. Hand-stand training is standing with hands. Right that? I am not clear the relation between hand-stand training and grip strength. Because when you use hand to stand you use your palms and I am a little confused. Anyway, there must have some theories that I don't understand. Please explain me. And is there any other grip strength training which is easier and more effective than hand-stand training? Thank you.

There are dozens of grip training exercises. Pull ups, and hanging from a rod are considered good, as is the actual gripping of objects.

The hand stands increases balance and concentration, both of which are important.
The hand stand, would increase general arm muscle strength

For hand stands look up cambered hand stand and/or training.