This is the second safest way to kill Zombies using the Knife in my opinion! Simply walk backwards as a Zombie is approaching you and stab its feet (simplest and safest approach) as you retreat. The best method is to stab it once, retreat and do it again. Practice this process regularly and try to incapacitate the Zombies quicker each time. It takes practice, but
perfecting this method will result in far less consumption of ammunition (works wonders in Hard Mode).

Try to avoid using this method in thin halls unless those halls are lengthy!