The other two power sources are also explained, but in less accessible terms. Apparently Earth power comes out of the ground, and can be substituted for Heart and vice versa in a pinch, although the greater the amounts of power involved, the riskier it is to make such substitutions. Sky power comes from the sun, or (rarely) the moon or stars, and is similarly not-quite-interchangeable with Will.

There are further explanations of how to connect to each of these power sources, starting with Will, which is apparently the easiest; the caster need only focus very hard on the desired goal, and the power will become available to send through the Dream-gate. Similarly Heart, using one's own emotions, is just a matter of keeping the emotion and the spell in focus at the same time.

Using someone else's emotions is trickier. Apparently you have to touch their mindscape. There is a separate chapter on how to do that.