From Diablo Wiki
The ability to knock back a target preventing it from attacking you. Uniques and bosses cant be knockbacked. Some skills are not great along with knockback, zeal comes to mind, as the first hit, would prevent the 4 follow up hits. Other things are great along with knockback, here the ability to stun comes to mind, this will effectively put enemies out for a short duration of time, each time you hit them.