Aún no tenemos significados para "feel a slackening".
1Suddenly he felt a slackening of the rope, accompanied by a faint sickening sound.
2But now she felt a slackening of the strain.
3Rafael felt a slackening of his powers of endurance, as if an irresistible languor were stealing over him.
4"And your musculature is responding well; I can feel a slackening of tonus already."