In criminal investigations, a holdback is a piece of information about a crime which is known to police but not released to the public. The purpose of a holdback is to help police recognize false confessions. Common holdbacks include specific details of the modus operandi and the appearance of the crime scene.
On a carriage, a holdback is a device, such as an iron catch on the shaft with a looped strap, that enables a horse to back or hold back a vehicle.
References
Holdback Wikipedia(Text) CC BY-SA