If you had

a factory that you wanted to protect from robbers,
the most sensible thing to do would be to place a fence
around it to make it a hard target. Therefore a potential
robber has got to get past that fence before he can even think
about attacking the factory. Whilst the fence might not keep
him out indefinitely it will make his job decidedly harder.
Rather like a boxer who constantly flicks a jab into his
opponent's face, even if that jab does not hurt his opponent it
keeps him at bay. If his opponent wants to employ his
Knock Out blow he first has to find away past his opponent's
jab-to the boxer the jab is the fence around his factory.
In practical terms the 'fence around your factory' is your lead
hand, placed in that all-important space between you and
your antagonist to maintain a safe gap.