asana involves some or the other action. But the asana
where you dont do any action is called as Shavasana.
There are three important components of Human existence,
these are body organs, energy supply and mental activity.
In Shavasana, one has to stop or reduce the activity
on all these three levels. He has to stop or reduce
his physical activity, reduce the energy requirement
by reducing the strain and also reduce the mental activities.
Stopping the physical activity is somewhat easier but
stopping the mental activity is extremely difficult.
Hence the Shavasana is difficult.