What is a Geas and how does it work? In specific, how does a Pysad use it?

A geas is like a pact you sign. It stipulates the conditions under which the magic point works.

For adepts this also means abilities tied to that point. A geas can be something such as "only from the end of winter to the beginning of fall" or maybe one month at a time, or whatever.

A Physad had to fulfill the geas (or stay within the limits it sets) in order to make use of the abilities tied to that point. Break that geas? You lose all powers connected to it and may have to atone, it depends. Most of the time you'll probably just lose qualifications due to time and have to wait until the next opportunity comes up.

I think adepts can also take a physical focus which, if always kept in close contact with the body, can be used to less the cost of the geas karmically or fulfill it entirely.

I haven't read MITS in months, but I think that's how it generally works.

It's basically a limit on your magical abilities and how you work magic. Mechanically, it allows you to retain some magical ability you may have lost due to wounds, cybernetics, body index, and assorted essence loss. Adepts can do this as well as any magical character.

Gaes are basically conditions which say you have to gesture using magic or something like that. If you don't, your magic is less and it's a cumulative +1 to magical tests.

Adepts get a bonus in that they can apply geasa to their powers, making them only cost 75% of the original cost. The downside is that they can't use their powers if they can't fulfill the geas.

Check out pages 31-33 in MitS.

Don

Both these guys got it right, but there's some clarification needed:

A geas is a restriction on your magic. If you break it, your magic becomes restricted. That much is pretty straightforward. The more frequent ones are condtion (must be sitting in full lotus, must be night, and so on); Talisman (must have particular lucky charm); and Performance (Incantation, gesture, or dancing).

Geasa can be obtained in one of two ways-- voluntarily or involuntarily. Involuntarily means you lost a point of Magic, and opted for a geas to offset the loss. Voluntary means you chose to take it, in return for additional power (either as a Ordeal for initiation, or for reduced costs on adept powers). You can try to shed involuntary geasa on initiation (restoring the lost magic point) but you are stuck with voluntary ones.

If a full mage breaks a geas, then he's at a significant penalty until he can refulfill them. In fact, if he has multiple geasa and breaks even one, he's at the full penalty until they're all met.

Adepts do not have this issue. When an adept breaks a geas, he simply loses access to the powers tied to said power point. All his other powers are unaffected. For example: if an adept has the voluntary geas: Gesture tied to his Distance Strike power, if he breaks his arm, he cannot use the power. However, his ungeased Improved Unarmed combat as well as his Pain Resistance power are still in full effect; so he can still fight with minimal penalties.