CommandSpirit, 4 points/level, Charisma/Hard skill.  (SF/Spirit defaults to Command -2 for spirits within your SpiritSpecialization)

CommandSpirit is used to communicate requests to spirits.  The sorcerer makes a request and rolls a quick contest of his CommandSpirit skill, modified by how bizarre/complex the request is. If the sorcerer wins, the spirit is beholden and will perform the task, departing when it is done.  If the spirit wins, it is uncontrolled and may negotiate with the sorcerer, attack him, or simply depart.  Spirits often seek remuneration for being summoned.  Stalling is a valid tactic—multiple command rolls can be made on a single spirit, although this takes time.  Use of the skill in this manner is only effective against spirits in the SpiritSpecialization.

Command can also be used on spirits controlled by others.  By making a request and concentrating for one second, the sorcerer causes a quick contest between the spirit, the sorcerer the spirit is currently beholden to (remember, he may have a bonus to his skill) and the WL of the spirit.  If the spirit wins, it becomes uncontrolled--otherwise, the sorcere who won is owed one task by the spirit. "Go free" is a valid request, and due to it's simplicity gives a +1 to effective skill.  Spirits outside the SpiritSpecialization may only be commanded to go free, and instead of the usual +1 this is at a -10 to skill.

The enhancement Spirit Liberator reduces the penalty to free a spirit from its command by 5, and can be taken twice.  It is a 10% enhancement on the SpiritPower, or a 25% enhancement on just the CommandSpirit subpower.

Spirits cannot be commanded to depart—they may be set free by the sorcerer, but they then do as they decide.

Remember that all command rolls for a spirit you have summoned receive a skill bonus of half the success margin.