How can a warrant be used to arrest a person?

In Arrowsmith v. he Mesurier, above referred to, Mansfield, C.J., said: I can suppose that an arrest may take place without an actual touch, as if a man be locked up in a room; but here the plaintiff went voluntarily before a magistrate. The warrant was made no other use of than as a summons. The constable brought a warrant, but did not arrest the plaintiff. How can a man’s walking freely to a magistrate prove him to be arrested. I think that the jury have done justice.