Prisoner Shackles are automatically equipped on a character when they are enslaved. Any gear in the "Boots" slot will be placed on the floor. Shackles weigh 20kg so that slaves are weighed down and run slower.
Shackles can be removed by Lockpicking, using Tools, or broken off using Strength. These items have a lockpicking level of 20 and a strength level of 20.
Equipping these on a character will immediately give that person the Slave status. However, they are not considered your slave, but usually a slave of the faction which the Shackles originated from.