(copied from elsewhere, and modified for correctness)
You can create special effects using SPECIAL EFFECT - CREATE SPECIAL EFFECT AT POINT
and SPECIAL EFFECT - CREATE SPECIAL EFFECT ON UNIT.
If you create it on a unit, you have to specify where on the unit (attachment point). This is a list of the available places:
overhead (floats over the unit's head, but doesn't sway with the head as it animates)
head (sways with the unit's animation)
chest
origin (usually at the base of a unit's feet)
hand
foot
weapon (for heroes)
sprite (for buildings)
medium (for buildings)
large (for buildings)
There are also attachment point modifiers. You can combine them with the attachment points above e.g. left hand and second sprite. Some combinations may work, others won't. This is because the attachments are totally unit/building dependant, the model itself must have the attachment point defined on it, else trying to attach something to that point will not work.
left
right
mount (for mounted units)
rear (for quadrupeds)
first (for buildings)
second (for buildings)
third (for buildings)
fourth (for buildings)
fifth (for buildings)
sixth (for buildings)
rallypoint (for buildings)
Cool example:
you know the spheres that hover around the blood elf hero? Those are actually attachments points that are set to move around the model in that fashion, and the spheres themselves are simply attached to those points. The points (off the top of my head) are first, second and third sprite. Try attaching something to them and see if it works or not.