Hello.

As you may have noticed, ice storm damage is applied in ticks and occasionally two ticks will trigger on a target as it passes by.

I have been unable to consistently work out how to trigger this tasty bonus, though I have had best luck with two techniques: 1) stepping back while casting, thus minimising relative speed of Ice storm to the target, 2) firing the ice storm at a non-horizontal angle, thereby increasing the distance through the target. These combine to increase the 'time on target' but still without consistently being able to cause the bonus.

Any other tips or observations?

Maybe a quick Fus shout. Just the first word of it.
It might stagger them back into it. I haven't tested it out, but just guessing.