here is a quote from Harold McGee's book "On food and cooking. The science and lore of the kitchen." from the section about tempering of chocolate:
Unstable cocoa butter crystals ... melt ... at relatively cool temperatures, between ... 15-28C. The desirable stable crystals ... melt only at warmer temperatures, between ... 32-34C. The temperature range in which a particular kind of crystal melts is also the range in which it forms as the chocolate cools.
As I understand, this means that crystals form just below their respective melting points when the chocolate cools. Then, if we temper by tabling (from "scratch" so to speak), why do we cool chocolate to much lower temperatures than just below 34C, which lets the unstable crystals form, and then we melt them by raising the temperature again above their melting point (but below the meting point of stable crystals)? If the stable ones form as the book says then it would be sufficient to cool to the temp where they form and not bother with cooling to the point where all others are formed? I am confused...