I agree with cordoval, the explanations are very clear... 
Maybe you could put it here something like 

```php
// You can do here whatever you want on your resource by accessing it with 
// $event->getSubject();
$resource = $event->getSubject();

// You can also here stop the propagation of the event before changes being effective in the database
// explain message and flashbag etc..
$event->stop('app.error.message', array('%my_param%' => 'param_value')); 
```

ping @amenophis  @cordoval