Sweeping Fend - APG Fighter Option. How does this work?

[jstbell]

Sweeping Fend (Ex): At 13th level, a polearm master
can use any spear or pole arm to make a bull rush or trip
maneuver, though he takes a –4 penalty to his CMB when
making such attempts. Weapons with the trip property
do not incur this penalty on trip maneuvers. This ability
replaces weapon training 3.

Is this a free trip attempt? Like as part of an attack? Cause I don't understand how it would be useful otherwise.

[Glutton]

Unless it is described as a trip weapon, you make your bull rush or trip attempts unarmed. It is commonly overlooked.

Some advantages: You can do it at reach with a reach pole or spear, thereby avoided attacks of opportunity on opponents without reach; if you have a +5 spear or pole you actually gain a +1 to your cmb (+5 - 4 = +1)

[HaraldKlak]

And you get the bonus from weapon focus, weapons training, etc.

On top of that, the ability is a great way to use those maneuvers without he required feats. If the enemy does not have 10 ft. reach, he cannot make AoOs against you.