The success of a bash depends upon how well you are learned. To bash someone is to attempt to 'knock' an opponent over with the force of your shield. If you succeed you will inflict minor damage, and cause a creature to fall on its rear. A creature will be stunned and unable to do anything for a short time. The amount of time depends on the creature's level and dexterity. If you miss you lose your balance and fall. You are unable to bash something more than twice your weight. It takes time to recover from a bash attempt. The amount of time is dependent on your level and dexterity. All things equal, it takes slightly more time to recover from from bashing someone than it does from bashed.
Note that a person who is not standing will suffer more damage than a person who is standing.