The act by which something slackens; loss of speed, tautness, etc.
1861, United States. War Dept, U.S. Infantry Tactics (page 124)
If the following guide lose his distance from the one leading (which can only happen by his own fault), he will correct himself by slightly lengthening or shortening a few steps, in order that there may not be sudden quickenings or slackenings in the march of his platoon.