Come on Fred, who doesn't like some hot lil blue dancin ladies
I'll throw in my opinion on the whole thing, cuz I've wondered the same. Bear with me, there is a lot going on here... It seems to me that if the coal bed is under a particular temperature, there are no flames and any over the fire air will result in wasted heat going up the chimney, although you may need this heat to help support your draft. BUT, if it gets above a particular temperature, there is enough combustible gases coming off the top of the coal bed to support flames when air coming in over the fire is introduced (like you've noticed). There are a couple things that can happen at this point.
#1 - These gases, without any over the fire air will simply float up and out the chimney. (wasted potential heat)
#2 - These gases, without any over the fire air could build up in the fire box and flash. (puff back)
#3 - These gases, WITH just the right amount of over the fire air, will yield me a few more BTU's
Number Three is tricky.. Meaning too much over the fire air will net you lost heat going up the chimney (in the form of heated air) and at the same time, not enough over the fire air will also net you lost heat going up the chimney (in the form of unburnt combustible gases). SO in my opinion, the perfect case scenario is the right combination of under the fire air (primary) and over the fire air (secondary) and that these air feeds are completely independent of each other. Meaning, primary air can't bypass the coal bed and act like secondary air, which was a huge problem for me. This combination will change based on just how hot you are pushing the fire. This is based on my own thought and observation. I would love to hear some others' thoughts on this also! Results and opinions may vary, some side effects could be life threatening (probably not) or are just a lot of fun to watch (Fred)