よそから参照されていない、weak参照のプロパティは即座に解放されるという話。

weak.swift

import UIKit

class ViewController: UIViewController {
    var presenter: Presenter!
    func inject(presenter: Presenter) {
        self.presenter = presenter
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        presenter.viewDidAppear()
    }

    func transition() {
        present(UIViewController(), animated: true)
    }
}

class Presenter {
    weak var view: ViewController!
    weak var router: Router! // <- weakでもつ
  　　　　// ↑はweak属性なしでよい

    init(view: ViewController, router: Router) {
        self.view = view
        self.router = Router(view: view) // <- Warning: Instance will be immediately deallocated because property 'router' is 'weak'
    }

    func viewDidAppear() {
        router.transition() // <- Error: Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value
    }
}

class Router {
    weak var view: ViewController! // <- weakでもつ
    init(view: ViewController) {
        self.view = view
    }

    func transition() {
        view.transition()
    }
}