Let me first recommend two books that may help clear things up. The first is called the Airplane Flying Handbook. It is an FAA publication, and is basically the final word on all the "whys and hows" of each maneuver. It's invaluable, in my opinion. The second is the Gliem CFI Maneuvers manual. It has everything that the AFH has, plus information on the FOI, etc. It's basically a super-elaborated CFI PTS. It's very helpful when creating lesson plans, and learning the maneuvers.
About Eights on Pylons--They are perhaps my least favorite maneuver ever. I personally find Lazy Eights more simple, and easier to explain. It was the worst maneuver on my CFI checkride (but I still passed).
The crux of the manuever (for me) seems to be remembering which whether to add or release elevator pressure to keep the wing on the pylon.
Anyway, I'm sure you remember how to peform this maneuver yourself. The challenge with teaching it being able to talk as you perform it--and be able to include enough information, yet also being able to select the information that you do give. Sometimes too much information in flight can be just as bad as too little. The AFM and Gliem will give you good ideas of what you could say to a student to help the transfer of knowledge.
Also, on the CFI checkride, the examiner will take the role of a commercial student (during the commercial maneuvers, anyway), so you can be a bit more technical in your descriptions.
"I'm not sure I understand the "why" of it however...?"
It's really a lot more simple than you are making it. In fact, you described it perfectly--a higher groundspeed requires a higher altitude. And that's all you want to say in the air. (On the ground, you can get into technical descriptions of "why," i.e. the mathematical formula etc. In fact, I'd have to look up the exact reason of "why," I've never gotten asked that. All my books are packed away though. Those two recommended manuals should shed some light on the subject.)
Hope this helps.