Pressure triggers are similar in concept to compression triggers; in fact, pressure triggers can be based on a bellows compression trigger, although they can also be based on levers. They detect a change in pressure, at which point the trigger is activated, creating a change in a linked object (releasing a catch, firing a crossbow, or producing some other result.)
The simple lever pressure triggers involve a plate or piston adjacent to a lever. When pressure is added, for example by being stepped on, the plate moves the lever, triggering another action. Placing a spring between one end of a lever and a surface that it moves towards when activated means that the lever will return to its original position after pressure is released; this can also be done by attaching a spiral spring in its relaxed state to one end of the lever and the fixed surface it is closest to, so that when triggered, the spring will be stretched, snapping back when the pressure changes. One use for this is a repeating pressure plate which automatically resets when the trigger is no longer activated. Another use is for pressure plates that detect the removal of weight (the idol that triggers a trap when removed from the pedestal.)
A hydraulic pressure trigger uses a fluid that can't be compressed, like water, to transfer pressure from one pressure plate or piston to another. A pipe or channel of any shape or length is filled with water and a loose plug is placed in either end. Pressing one plug in causes the water to push the second plug out, and vice versa. If there is a lever or spring trigger in contact with one plug, the movement of the plug will move the trigger. This is a great, simple way to transfer one trigger effect to another location, especially if a rope and pulley system would be too complicated.
Another use of the hydraulic pressure trigger is to leave one end uncapped, so that the fluid is squirted out of its tube, perhaps creating a slick area or squirting flammable liquid into an open flame. However, this use is perhaps closer in form to a pure compression trigger, instead of a pressure trigger.
Either kind of pressure trigger can have horizontal plates or pistons instead of vertical ones, creating a simple button. This is mostly of use to trigger secret doors, puzzle locks, or to disengage another trigger, disabling a trap.
Pressure triggers are detectable in the same way that compression triggers are: visible difference in coloration or texture between the plate and the surrounding floor or wall, gaps around the plate, plate protrudes slightly, or is recessed slightly into the surface. Tapping a pressure plate may detect a hollow area, but you might not want to tap it, if it is very sensitive. If the mechanism behind the surface is large, the hollow area may be larger than the plate, which means the hollow is detectable by tapping around the plate. A hydraulic trigger might have some leakage, plus the pipe or tube system might be traceable by careful tapping.
Pressure triggers are hard to jam, although it might be possible. The usual procedure is to avoid them or deliberately use a pole or rolling object to trigger them from a distance, although the latter doesn't do much good if it's a repeating pressure trigger. If the trigger detects the removal of pressure instead of an increase in pressure, another object of the correct weight may keep the trigger from activating.