Mr Covey’s ability at stealth affects the slaves' work habits in his absence by making them less productive.
What was the effect?
Knowing Mr. Covey could appear unexpectedly and without warning could create a constant state of fear and anxiety among the slaves. This fear could make them hesitant to work efficiently, knowing they could be punished for even the smallest perceived lapse.
Mr. Covey's stealth also fostered a lack of trust between him and the slaves. They felt he was always watching them for an opportunity to punish, so they became less cooperative and more likely to slack off when he wasn't around.