If you want to save someone from sliding off, you have to pull them before the knockback registers. I'm not 100% on whether knockback cancels a rescue pull. If it does and they're only halfway to safety, it might still kill them. If not, it would either give you some wonky motion effects as the player teleports to you faster than they should, then gets pushed, or the knockback would just appear to knock them back less distance than everyone else.