This loop thrives on a brilliant parasitic relationship between Devoted Druid and Quillspike. By tapping Devoted Druid for mana and placing a -1/-1 counter on it to untap itself, you generate both the green mana and the exact fuel Quillspike needs. You then use that generated mana along with Quillspike's activated ability to remove that same -1/-1 counter from Devoted Druid, which conveniently pumps Quillspike into a massive threat while resetting the druid to do it all over again.