The expression will-power is often used to refer to our ability to put into practice the ideas we know to be for our well-being and to resist actions which are harmful. This is directly related to the soul’s intellectual strength. When we speak of weakness or strength in the soul we are referring to the intellect. In the case of a weak soul (one with lower will-power) it is almost as if the intellect plays no part in determining which thoughts arise in the mind, but they come as if pushed by the sanskaras (mainly in the form of habits) or are triggered by the atmosphere around or the moods of others. On the contrary, a powerful soul (one with higher will-power) enjoys the experience of its own choice regardless of external stimuli (influence).
Rajyoga meditation develops the intellect to such an extent that this degree of control is possible. A practitioner of Rajyoga meditation can be in the midst of a situation of intense disturbance, yet remain so unshakeably calm that the inner strength becomes a shelter and inspiration to others lacking in that strength. The weak soul is like a leaf at the mercy of the storm, the strong one, a rock in the face of a rough sea.