A piggyback referred to a condition where one thing was done as a part of something else, in which the secondary task was carried as overhead to the primary one. This could occur in regards to a computer system piggyback.
Data, in 2368, with the assistance of Captain K'Vada, accessed the Romulan Central Information Net to send a transmission to the USS Enterprise-D, piggybacking it on Romulan transmissions. K'Vada was initially confused by the Human metaphor when Data used it. (TNG: "Unification II")
In 2371, B'Elanna Torres postulated they might be able to transport the crew of USS Voyager back to the Alpha Quadrant by piggybacking a transporter beam onto the visual link of Telek R'Mor's signal. According to Kathryn Janeway, it would have necessitated re-configuring the matter transmission rate. There was also the likelihood of losing that which they attempted to beam away. It was agreed that tests would be carried out to determine the plan's feasibility.
After adjusting for an unexplained phase variance, they managed to successfully transport a test cylinder to R'Mor's ship over twenty times. They then beamed R'Mor himself to Voyager in order to discuss plans to rescue the ship, only to discover that the reason for the phase variance was the fact that R'Mor was from twenty years earlier than they were. As a result, their plan was rejected, as they would wind up twenty years in the past and would thus contaminate the timeline. (VOY: "Eye of the Needle")