It's sort of complicated and you may just need to do some PvE to really get the hang of it. But basically you use Slam over HS whenever it won't interfere with your GCD. If you're >80 rage though, you typically want to use HS to prevent any rage gains from being completely wasted.
Sometimes though, you can try to stack Taste for Blood procs (up to 3-5) for a strong burst window -- not worrying about excess rage or mediocre damage in the meanwhile -- but that requires some RNG and extended uptime on a target. Also, if someone is 40% HP and you're 100% rage, you'd obviously want to be using both Slam and HS for the quickest short-term burst, while saving 30 rage for a potential execute.
And finally, if you're the one on your team responsible for keeping a target snared, you may want to consider using Glyph of Hindering Strikes, and using HS more as a hamstring replacement. You will still use Slam with this glyph, but you'll be spending more of your rage on HS and usually won't be going for Taste for Blood stacks.
It appears to be that, from time to time, three wizards who can cast healing spells join forces and cause quite long and boring matches. However, I find it easy to tolerate the superfluous length when I occupy myself with tea and crumpets.
I do concur that the act of wizardcleaving is indeed an art of great skill.