You are right. A maximal voluntary contraction is essentially your 1RM. An involuntary contraction makes use of the stretch-shortening cycle and the reflexive properties of your connective tissue (tendons, etc.).
An example of this occurs in sprinting. Elite sprinters absorb impact forces off up to 5 times their body weight (I think I am recalling that fact correctly) on one leg during top speed. Obvioiusly they wouldn't have a chance in hell of 1-legged squatting 1000 pounds!
They are able to absorb this because of their incredible reflexive and elastic abilities. That's the difference between maximal voluntary and maximal involuntary.
I think I read that part of Supertrianing right??