To use SQL syntax with model, that would transfer result to proper instantions, you should use directly one of Phalcon\Mvc\Model\Resultset classes:
$users = new \Application\Models\Users();
// bitwise operation on `flag` field
$sql = 'SELECT * FROM phorum.users WHERE
(15 & (1 << (flag - 1))) > 0 ORDER BY login DESC';
// as a result you will have a Resultset\Simple with Models\Users instances.
$result = new \Phalcon\Mvc\Model\Resultset\Simple(
null,
// what model to use for data returned from SQL
$users,
// setting result via "read connection" proper for this model.
$users->getReadConnection()->query($sql)
);