A sigil is required to send any being to the alternate Earth. Balthazar used salt, blood of lamb and a bone of a lesser saint for the ritual and the sigil. Any being that made contact or was forced to make contact with the sigil in the immediate vicinity of the sigil is then drawn into the alternate reality.
The spell could not be reversed simply by performing it again. It had to be performed in the original reality, and as it was Raphael who planned to draw Virgil out of alternate Earth, the reversal of the spell may require a more powerful being to cast it successfully. Raphael's reversal of the spell utilized the same sigil, although it glowed bright red before it drew Sam and Dean back into their original reality.