Gestures, and how they make up Spells
On the Spell Reference Page, you'll see that each spell has a sequence of Gestures associated with it - quite a long sequence, in some cases.
"How," you may ask, "can I cast one of those Spells when I can only make two Gestures in a Turn?"
The answer is, you have to make the Gestures of a Spell consecutively, through a series of Turns.
There are 8 possible Gestures:
The abbreviation for each Gesture are used when specifying Gestures for a Turn.
Note that a single Gesture can be used in more than one Spell. For example, the consecutive Gestures PSDD will cast three spells - P alone is a Shield spell, SD in the middle is a Magic Missile spell, and PSDD is a Charm Monster spell. All of these gestures would, however, have to be made with the same hand. Also note that a single Gesture cannot be used as the final gesture of more than one Spell, eg. SPFP is Anti-spell; you can't use the final P gesture to cast both Anti-spell and Shield.
Similarly, in the two-handed sequence
You could not cast both Invisibility (PPws, in which both the w and s must be two-handed), and Counter Spell (WWS on one hand) in the final Turn, even though the Gestures are there for both Spells - both hands' "S" Gestures would be used for Invisibility, so you must choose which spell you want to cast. You don't need to choose a spell until the turn on which it will be released.
If a caster completes a series of Gestures that make up a Spell, intentionally or unintentionally, the Spell must be released, though it may be cast at nobody. This is important when the Spell is one such as Fire Storm, which affects Warlocks even if cast at nobody. Also, if a Warlock doesn't notice they are casting a Spell, they will unintentionally cast it at the default target, where it may have an undesired effect.
A spell can be aborted at any time before its final Gesture, simply by making different Gestures - there is no penalty but the wasted time. Note that making a 'nothing' gesture does not simply delay the casting of a spell - it breaks up the spell. SPF--P will not cast Anti-spell (SPFP).
Note also that a Warlock cannot stab with both hands simultaneously. A stab Gesture can be made with both hands, but if this occurs, only the right hand will do damage. (Changing hands between turns is no problem, however.)
Important: If you make two 'P' Gestures in the same Turn, you surrender, whether you intended to make the Gestures or not. You can be made to surrender by Paralysis, Charm Person, or Confusion (and bad luck), or by your own inattention. Sometimes, surrendering is a good course of action - see the rules on Scoring.
Next: Winning and Scoring