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?