This is pretty straight forward:

1. Attack a mob that is aggressive towards you

2. Notice that while they are being damaged. their head will stop looking at you, they'll stand still, and stop attacking. After about a second they will re-aggro.

In 15w31c, this can be more easily seen against skeletons because of their new animations:

1. Attack a skeleton that is aggressive, they will being aiming their bow at you

2. Notice how after you attack them, they put their bow down, stop looking at you, and stop firing arrows. They will re-aggro and redraw their bow.