A small pump piston drives fluid into a much larger ram cylinder. Pressure is the same throughout, so force is proportional to area - the larger ram produces proportionally more force. The cost is distance: many strokes of the small piston move the large one only a short way.

Usually decided by geometry rather than capacity. Work from the clearance beneath the load, the lift required, and where the jack can bear on the load. A bottle jack needs room for its whole body; a toe jack lifts from near floor level; a low-profile jack exists for restricted vertical space.

No, and this is the one rule that matters. A jack lifts; it does not support. Hydraulic pressure is retained by seals, and seals fail - so axle stands, blocks or a mechanically locking jack must take the load before anyone goes near or under it.

The overall height of the jack fully retracted, which determines whether it will fit under the load at all. It is the specification most often overlooked, because buyers check capacity and lift height and then discover the jack will not go into the space available.

It means the jack is rated for that force, but the load must also be stable, the ground must take the point load, and the lifting point must be sound. A jack at its rated capacity on soft ground or a corroded lifting point is a hazard regardless of the rating on the label.

Checking oil level, inspecting for leaks past the ram seal, and keeping the ram clean and lightly oiled so grit does not score it and destroy the seal. A jack that creeps down under load has a failing seal or valve and should be taken out of service rather than watched.

Usually low oil, or air trapped in the system after transport or a seal change. Bleeding the jack and topping up to the correct level fixes most cases. If it still falls short or feels spongy, the internal valve or seal is at fault and the jack needs repair or replacement.