When he talks about movement mitigation... he means developing the ability to assess incoming movement needs.. and moving the absolute minimum required to avoid the threat and maintain maximum uptime of casting.
Sometimes it's moving just a hair out of the way and resume casting... other times it's knowing that moving a little more NOW will save you from moving again in just a few seconds. Knowing the fights intimately is part of it.. but having a "knack" for assessing things quickly and making snap judgements correctly.
We've all had those "in the groove" days where we feel like we play in "bullet time" and our choices are perfect, movement precise and damage over the top.
I am just getting somewhat comfortable with the El. Shaman abilities and priorities. The rhythm of abilities feels very similar to that of my mage.