Pinching is plucking out the growing tips before they lignify (begin to harden off) while they are still succulent. it reduces leaf size and can be applied during any season in which the tree is actively growing.
Please see this thread for an article about pinching
http://www.ip-sys.com/~bonsai/forum...+japanese+maple
It's a red maple in the article. Red maples seem to be less vigorous than your garden variety green japanese maple, but they can certainly be pinched.
The other technique you discuss is called defoliation. There is an article on that on the startpage at bonsaiTALK.com
Regards,
Matt