Under the bill, the test with respect to the preventative arrest for the purposes of attendance before a judge is that the police officer has to believe “on reasonable grounds”--again--“that a terrorist activity will be carried out”. But what's different from the current law is that there may not be reasonable grounds to believe who the perpetrator is. So that's why this fills a gap.
With respect to the second stage, the bill provides that the police officer has to have reasonable grounds to suspect that arresting a person will “prevent” the activity. That doesn't mean that the person who's arrested is the perpetrator. It may be other individuals who are involved.
For example, the scenario that has been asked about before is where this provision would be used. One can imagine a situation where, say, there's a demonstration, the police have reasonable grounds to believe that a bomb is going to be detonated during the course of the demonstration—not by the demonstrators, but by other persons—and the actual perpetrators are not known. They know it's going to happen as a result of intelligence. But they also suspect a number of individuals who have been agitating, who have been making statements, and they may have reasonable grounds to suspect that these individuals know something, that they have assisted others, and by bringing them before a judge, that does two things: one, it puts them under the judicial control of the judge; and two, it also sends a clear message to others that the police are aware of a conspiracy or possible attempt. It's a way of publicly indicating that they know something is about to happen.