A hacking movement is one in which the movement can be stopped by hand.
Also called stop seconds, hacking movements became popular as precise watches and timekeeping appeared in the 20th century. During wartime, hacking was a desirable feature since it allowed soldiers to "synchronize their watches", a common moment in many war movies. With the advent of chronometers and increased focus on the accuracy of watch movements, hacking is an important part of testing and adjusting watch movements.
Many movements can be made to hack by pulling out the crown. This causes a "hacking lever" to interrupt the balance wheel and thus the movement of the gear train. Some watches can be made to hack by exerting pressure on the crown, increasing friction at the hour hand collet, but this is considered harmful to the movement and should be avoided.
Non-hacking movements can be set to the second by allowing them to run down and stop, only winding and restarting them at the appropriate moment. This is difficult, however, and is not as precise as a true hacking movement.