KeepSprint != AutoSprint
It's a vanilla behavior in minecraft for you to stop sprinting for a split second when attacking a mob. Autosprint makes you instantly sprint after it, but there's still a small delay.
If you hit a person you're chasing, but your attack doesn't go through, it'll cause you to slow down with no benefit of hitting them.
Using AutoSprint will help you keep sprinting.. but it's absolutely no different than just holding Ctrl.
KeepSprint will remove the small sprinting cooldown, so when your hits don't go through you can keep chasing them.
Of course, if you hit a person and they can't be knocked back as fast as you're chasing them, you can essentially keep hitting them over and over without slowing down for an infinite combo. This works really well in 1v1 with a knockback sword.