/stopcasting

[Klaus]

I'm fairly certain that this is what happens:

You cast smite
Start casting again with /stopcast; note this smite has a casttime
1st Smite lands - triggers a proc

When your 2nd cast completes, it "uses" the charge even though it wasn't instant. Basically your charge is completely wasted.

I suppose that if you notice the proc during the 2nd cast, you could just whack the key again, which should cancel the one you're casting (the one with the cast time) and restart it - which should then consume the proc and be instant cast. So, you'd have to be on your toes about using the macro to compensate for the lag, and also to maximize your use of SoL procs.