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?
I have no knowledge of Thai but what I think is said here is:

Anatta nature is such that you cannot effect any of the aggregates by will power alone in an absolute way ever lasting way. You can stay in one posture but this is to a limited time. So you have limited influence but not absolute at a grosser level. At the level of arising and passing you have no control, whatsoever. When phenomena arises you cannot make it not pass away. You cannot stop phenomena arising.

But having said this you can have limited influence over all the aggregates by what you do at the present moment though influencing the fabrication process (Karmic and Citta Niyama resultants). E.g. by calming the breath you can calm the bodily fabrications. Certain volitions give certain results later. These can be viewed as regulating, i.e. you regulate the present moment. Awareness of the arising and passing away of: the aggregates / mind matter process / within the 4 foundations of mindfulness, equanimous to sensations devoid of clinging and craving, creating positive fabrication (like Metta), etc. are forms of regulation.