A strain gauge is a long length of conductor arranged in a zigzag pattern on a membrane. When it is stretched, its resistance increases.
Strain gauges are mounted in the same direction as the strain and often in fours to form a full 'Wheatstone Bridge'. The diagram above represents what might happen if a strip of metal were fitted with four gauges. An downward bend stretches the gauges on the top and compresses those on the bottom. A pressure transducer contains a diaphragm which is deformed by the pressure which can be measured by a strain gauged element.