Something that is piggy-backed (archaically known as pickaback) is something that is riding on the back of something else. To piggy-back (or to go on a piggyback ride) is to ride on someone's back and/or shoulders. People may give piggybacks to move people, to provide others with elevation or for entertainment.
Piggyback ride
One common piggyback involves the carried person sitting on the carrier's shoulders with each leg positioned on each side of the carrier's neck and dangling downwards. Alternatively the carried person is positioned lower laterally along the carrier's back, with their legs extending forward to provide balance and to possibly be supported by the carrier's arms. Another method of support for the latter method is for the carrier to stick his or her hands underneath the carried person's rear end. The latter position requires less physical strength and is safer for both persons, however by being positioned on one's shoulders the carried person's weight is evenly balanced and thus over time is less exhausting.
Other uses
This expression is used by analogy in several contexts, such as in ophthalmology, electrical engineering, computer engineering, computer science, structural geology and rail transport or intermodal freight transport. For example in space transportation systems, where the satellite is on the top of the launcher assembly, one says that the satellite is piggybacked on the missile. In electrical engineering a secondary integrated circuit may be piggy-backed onto a primary one e.g. to replace without soldering. If the second chip is mounted leads up, the primary IC is said to be dead-roached instead, since it looks like a dead roach with legs up. The term piggy-back is also used for a product distribution strategy, where two companies that have product lines that complement each other grant access to each other's distribution channel structures so they may both expand their markets (e.g. abroad). This is a variation to the principle of cross selling. In rail transport, the practice of carrying trailers, semi-trailers or containers in a train atop a flatcar (intermodal freight transport), see Autorack.
