Partial actions were originally part of 3e, but were taken out of the system due to some difficulties with complexity. However, with the existence of the templar, I feel that the concept of the partial action allays some amount of confusion in regards to jikuu.
Ergo, partial actions have been reintroduced in Trinity.
A partial action is, in essence, either a standard action or a move action. A normal character's round consists of two partial actions, up to one of which may be a standard action.
A partial action can be used to begin a full-round action; in a normal round, the full-round action is then finished with the other partial action.
Some effects, such as slow, reduce you to a single partial action each round. Other effects, such as hasteaga, grant you an additional partial action each round.
When you are disabled (at 0 hit points), you can take a single partial action each round without bleeding; if you are a member of a race that doesn't bleed, such as an artilect or adu'ja, then you can take a single partial action each round while at 0 hit points indefinitely.
Also note that standard actions can be used as though they were move actions, but move actions cannot be used as standard actions.
Gaining Partial Actions
A character normally has two partial actions each round; up to one of these may be a standard action.
When a character gains partial actions, they may be used for standard or move actions, at the player's discretion.