The following principles may be kept in mind when killing a group with a placement.
See: counting liberties
This is a standard life and death shape Carpenter's Square that if played correctly leads to a ko fight. Black's correct play begins with a placement, . The more liberties the harder it is to be killed. The aim is to live as long as possible. This principle not relevant when the placement is a sacrifice.
See: eye in the belly.
Obviously if you can make 2 eyes do so. But in a situation where the placement cannot live by itself inside the territory of the opponent's group, having an eye may decide the outcome of the situation. Using the previous example, suppose the opponent unwisely plays elsewhere:
threatens to create an eye at square (and in fact kills white unconditionally).
Using the same reasoning,
The correct is to attach underneath . This move seems unusual until one realise that
See: killing shapes.
Black is a bent four in the corner killable shape. If he plays at square in the next move he makes a bulky five which makes White dead when captured. However if he plays at circle his shape is unkillable when captured; white can capture it and live.
This simplistic diagram illustrates how, instead of looking for live shape, one wants the opposite (that is killable shape when captured) with regard to a placement group that cannot live on its own. e.g. straight three, bent four in the corner, bulky five... etc., so that the opponent is forced to capture, and ends up with a territory that is a dead shape. This does not necessarily mean death for the opponent's group, but it at least forces further play (e.g. escape into the centre, link up with another group, kill an outside group) to live.
Dieter: I don't think the discussion is as wrong as the remove label suggests, but it's more about killing from the inside, than about placement. Indeed, the content can be considered redundant with vital point, eye in the belly and nakade, which are more mature pages.
Removal is harsh, so I prefer keeping it as a subpage of the original author. Remember SL is not Wikipedia. On the other hand, the first version is 15 years old, so it probably doesn't matter too much.
hnishy: Principle 1 is meaningless, because placement doesn't touch other stones. Liberties are maximized by definition. Principle 2 example has many other ways to kill. Principle 3 is a general advice on inside plays and can occur from other moves like attachments or jumps too.