UIStoryboardSegue: top view is not sliding down

I have the following swift code and what i am trying to achieve is a segue that slides off the top. I want the secondVCView to be below the firstVCView and for the firstVCView to slide off displaying the secondVCView.

Currently there is no animation and it just flashes to the secondVCView.

Many Thanks

import UIKit

class TopToBottomSegue: UIStoryboardSegue {

override func perform() {

    // Assign the source and destination views to local variables.
    var firstVCView = self.sourceViewController.view as UIView!
    var secondVCView = self.destinationViewController.view as UIView!

    // Get the screen width and height.
    let screenWidth = UIScreen.mainScreen().bounds.size.width
    let screenHeight = UIScreen.mainScreen().bounds.size.height

    // Specify the initial position of the destination view.
    secondVCView.frame = CGRectMake(0, 0, screenWidth, screenHeight)

    // Access the app's key window and insert the destination view below the current (source) one.
    let window = UIApplication.sharedApplication().keyWindow
    window?.insertSubview(secondVCView, belowSubview: firstVCView)

    // Animate the transition.
    UIView.animateWithDuration(0.4, animations: { () -> Void in

        firstVCView.frame = CGRectOffset(firstVCView.frame, 0.0, screenHeight)
        //secondVCView.frame = CGRectOffset(secondVCView.frame, -screenWidth, 0.0)

        }) { (Finished) -> Void in
            self.sourceViewController.presentViewController(self.destinationViewController as! UIViewController,
                animated: false,
                completion: nil)
    }

  }
}

via Chebli Mohamed