Angular runs $parse automatically when it runs the $digest loop, basically $parse is the way angular evaluates expressions. If you wanted to manually parse an expression, you can inject the $parse service into a controller and call the service to do the parsing for you.

Here's a code snipped from ng-book that watches then parses an expression.

<div ng-controller=MyCtrl>

  <input ng-model=expr type=text placeholder=Enter an expression />

    <h2>{{ parsedValue }}</h2>

</div>

then in our module,

angular.module(myApp, [])

 .controller('MyCtrl',['$scope', '$parse', function($scope, $parse) {

    $scope.$watch('expr', function(newVal, oldVal, scope) {

      if (newVal !== oldVal) {

        // Let's set up our parseFun with the expression

        var parseFun = $parse(newVal);

        // Get the value of the parsed expression

         $scope.parsedValue = parseFun(scope);

      }

    });

 }]);