The book contains research done on Ayleid ruins at Cloud Top involving weather magic. Apparently the Ayleids possessed a magical method of absorbing lightning bolts and converting them into magical energy. This naturally required a magical device (the pillar) placed at a high point to absorb as many as possible.
"Fingers of the Mountain" is thus a poetic way of describing the natural lightning that would repeatedly strike such high locations. It should be noted that gaining the spell is likely a side effect of the original intent of the Ayleids as the pillar is broken and can not serve its intended purpose of drawing energy from lightning as a power source for other magics.