When a wizard casts this spell, he literally links another spell (that he must have cast himself) to a single word that acts as an "on/off" switch for the duration of that spell. The "password" that turns the spell on and off is picked during the casting of the Word Lock, and must be spoken by the wizard, but only sub-vocally (not really aloud; the wizard need not even move his lips).
There are some requirements on the sorts of spells that can be Word Locked, however. To begin with, they must have a duration other than instantaneous. Next, though, this duration generally cannot be permanent; thus you could not Word Lock a Transmute Rock to Mud spell. There are some permanent duration spells that can be word locked however; Iradel's Extra-Dimensional Denial is an example. Generally, if it cannot be Dispelled then it cannot be Word Locked. Non-permanent spells can, however, be Word Locked and then made permanent with a Permanency spell; thus, you could have a permanent Wall of Force that could be switched off and on. There are other spells that will not be logical to Word Lock, as well; spells like Dream and Magic Jar (both 5th level wizard spells from the Player's Handbook) just wouldn't make sense. In cases like this, D.M.'s should use their best judgment.
As mentioned, a Word Locked spell may be turned on and off with a word. This word, once again, need only be sub-vocalized by the wizard. Thus, even a thief's great hearing could not pick it up (as no sound other than breath comes out), and reading lips does no good (they don't move). However, mind reading spells might learn it, as might other means. Anyone who knows this word may turn the spell effects off and on, so long as they are within the range of the spell when it was originally cast (i.e., a 14th level caster may cast a Reverse Gravity spell from up to 70 yards away, thus it may be turned on and off from within that range). Spells that have a range of Touch may be turned on and off from within 10 feet.
Spell effects that are turned off completely cease to exist; all effects of them disappear instantly. When they are turned back on, they begin immediately, and reappear in the exact same place that they disappeared from. Let us look at an example. Let's say a wizard casts a Reverse Gravity spell into an area filled with orcs. They immediately start "falling" up into the air. The next round the enterprising wizard Word Locks his previous spell. . . and then shuts it off with his command word. The Reverse Gravity's effects immediately cease, gravity returns to normal, and the orcs fall. Then, as they lay groaning in the ground, the wizard proceeds safely through the area that contained his spell. . . then turns it back on with the same word. The orcs immediately begin to hurtle screaming back into the sky, and the wizard leaves. Finally, at the end of the duration of the Reverse Gravity, both it and the Word Lock will end, and the orcs will come hurtling back down. . . clearly this can be a deadly spell.
Spells may be turned off and on as many times as the wizard wishes, for as long as the spell that is Word Locked lasts. This does count as the wizard's action for the round, however, save for in the first round that the Word Lock is cast. In that particular case, the wizard may turn the target spell off immediately after casting the Word Lock. Otherwise, it is an action with an initiate modifier of 1 (or Fast in the Combat and Tactics rules).
Spells that are turned off still continue to use up their remaining duration; thus if a spell that lasts for 10 rounds is deactivated with a Word Lock after 5 of them, then left to sit "off" for 3 rounds, when it is turned back on it still has only 2 rounds remaining.