Why will-power could not be used to regulate kaya , vedana and citta?

I found the below from the book **Mindfulness with Breathing: A Manual for Serious Beginners** http://www.amazon.co.uk/Mindfulness-Breathing-Manual-Serious-Beginners/dp/0861711114 > The Thai word **kuab-kum** is used throughout these talks. It can be > translated **'to regulate**; to control or confine; to oversee, > supervise, or superintend.' When one of these translations appear, all > of the rest should be understood. In all cases, **kuab-kum depends on > sati and wisdom, never force or will-power**. Could someone explains why **will-power** could not be used to regulate kaya , vedana and citta?