To Will is the third cornerstone of the Powers of the Magus phrase. It is often linked with the Element of Water which makes sense because Water is associated with strong personal emotions and desires and going with the flow. It is about recognizing that much of what you want as a spiritual person involves some type of emotion. It grows into experiencing the feelings of connection with the energies you work with and a longing to connect with the Divine itself.

Knowing what you want and practicing to make it manifest through Daring allows you to add your personal will, i.e., emotions, to spellwork and magickal practices which deepens the experience and makes it more powerful. Your true will at its core is your spark of the Internal Divine which is the source of your magick and your own spiritual path.
Also think about how water just keeps flowing around the rocks of a river; it continues on its course despite any obstacles that it encounters. If finds a way to go around or over or under if necessary. It also eventually wears the rock away to nothing through the force of the current.

Similarly, putting your will into your spellwork allows you to keep going because you now have connected your knowledge and your workings to your emotions. Only when you are sure do you move on to the principle of the fourth cornerstone, To Keep Silent.