I was messing around on my druid casting wraths and I figured out a way to motion cast. Now I don't mean casting a full spell while on the run, but being able to run while casting the last 25%-50% of your spell (depending on cast speed).
For example, on my druid i was able to cast the last 50% of my wrath (1.5 sec cast) spell while on the run. I also tested this on a mage and was able to cast the last 25% of my fireball (3 sec cast) while running. ALSO WORKS WITH HEALS.
What you need to do is start casting your spell. and when it gets further than half you need to strafe left or right by holding down the right click and your turn left/right key at the same time.
This is NOT lag, I was doing this with 68 MS, and I tested this with running forward/backward and my casts were immediately canceled.
It's not full out motion casting but it can be very useful if you're kiting a mob/player and hes approaching you quickly while you're casting, or if you're in the same situation and trying to heal.
One last note is that if you receive spell knockback it will most likely add too much casting time and your spell will fail before it hits.
Done on latest ascent rev