This characteristic is like a grand archer testing a bow, or like tuning the peg of a lute so the tension of the string remains evenly balanced. Likewise, if one constantly guards mindfulness while meditating on bodhichitta and the visualisations of the developing stage, or while contemplating the essence of the pure nature of appearance in the completion stage, then one will have the sense of what is called samadhi by keeping mindfulness balanced at all times without intervals.

-- Thinley Norbu Rinpoche