Hold Position Command

The Hold Position command, often shortened as "hold".

This command can be shift-queued.

It makes sense to shift-queue this command as long as no move, attack-move or patrol commands are queued after it.

Once shift-queued a hold position to a patrolling unit, the unit will continue its way to the next patrol waypoint, discarding all other waypoints. Afterwards, it will stop in Hold Position.

Mutas vs Thors
This is the classic technique to fight Thors with Mutalisks. Use Move command past the Thors and when mutas are on top of them, issue the hold command. This will prevent mutas to stack. Using hold has the drawback that mutas that are not in range of any Thor will not engage them. Using stop instead of hold results in those mutas engaging as well but with the risk that they will stack - to the benefit of the Thor's splash damage attack.

Melee units engaging workers
This tactic allows melee units (e.g. Zerglings) to kill workers in the worker line without waste time chasing other units that would otherwise receive priority if the melee units were carrying out an attack move, patrol or stop command. When the melee units reach the area between the minerals and the base, issue the hold command.

Obstruction
One or two queens in hold position will block passage on ramp. Hold position guarantees that the ramp will be blocked unless they die.

Holding a Xel-naga watch tower
Holding a Xel'naga watch tower with a worker will cause the worker to be counted as idle, unless the hold command is issued. In general you may want to issue the hold position command to units holding Xel'naga watch towers so that they don't chase passing-by enemy units by accident.