Unit appearing right before another one/making tricky triggers

[TheMadman]

I've noticed very interesting thing in flame knives campaign.

In many levels, when you control graaka (defiler hero) you can control zerg units if graaka come near to them.

Or during boss fights, illusions always appear NEAR heros, not in specified location.

Its very interesting kind of triggers, and i am interested in them to make a campaign.

So, my question is: How to make those tricky triggers?

[Revolta]

Have a location centered around the hero.

Trigger action Move Location, make it move to [unit] at anywhere.
Preserve trigger.