MVC tutorial: I don't understand a syntax for Dependency Injection

I am using the tutorial below but I don't understand this syntax on the code:
public ContactController()
            : this(new EntityContactManagerRepository())
        {}
http://www.asp.net/mvc/tutorials/iteration-4-make-the-application-loosely-coupled-cs

using System.Text.RegularExpressions;
using System.Web.Mvc;
using ContactManager.Models;

namespace ContactManager.Controllers
{
    public class ContactController : Controller
    {
        private IContactManagerRepository _repository;

        public ContactController()
            : this(new EntityContactManagerRepository())
        {}


        public ContactController(IContactManagerRepository repository)
        {
            _repository = repository;
        }

        
        protected void ValidateContact(Contact contactToValidate)
        {
            if (contactToValidate.FirstName.Trim().Length == 0)
                ModelState.AddModelError("FirstName", "First name is required.");
            if (contactToValidate.LastName.Trim().Length == 0)
                ModelState.AddModelError("LastName", "Last name is required.");
            if (contactToValidate.Phone.Length > 0 && !Regex.IsMatch(contactToValidate.Phone, @"((\(\d{3}\) ?)|(\d{3}-))?\d{3}-\d{4}"))
                ModelState.AddModelError("Phone", "Invalid phone number.");
            if (contactToValidate.Email.Length > 0 && !Regex.IsMatch(contactToValidate.Email, @"^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$"))
                ModelState.AddModelError("Email", "Invalid email address.");
        }

        public ActionResult Index()
        {
            return View(_repository.ListContacts());
        }

        public ActionResult Create()
        {
            return View();
        } 

        [AcceptVerbs(HttpVerbs.Post)]
        public ActionResult Create([Bind(Exclude = "Id")] Contact contactToCreate)
        {
            // Validation logic
            ValidateContact(contactToCreate);
            if (!ModelState.IsValid)
                return View();

            // Database logic
            try
            {
                _repository.CreateContact(contactToCreate);
                return RedirectToAction("Index");
            }
            catch
            {
                return View();
            }
        }

        public ActionResult Edit(int id)
        {
            return View(_repository.GetContact(id));
        }

        [AcceptVerbs(HttpVerbs.Post)]
        public ActionResult Edit(Contact contactToEdit)
        {
            // Validation logic
            ValidateContact(contactToEdit);
            if (!ModelState.IsValid)
                return View();

            // Database logic
            try
            {
                _repository.EditContact(contactToEdit);
                return RedirectToAction("Index");
            }
            catch
            {
                return View();
            }
        }

        public ActionResult Delete(int id)
        {
            return View(_repository.GetContact(id));
        }

        [AcceptVerbs(HttpVerbs.Post)]
        public ActionResult Delete(Contact contactToDelete)
        {
            try
            {
                _repository.DeleteContact(contactToDelete);
                return RedirectToAction("Index");
            }
            catch
            {
                return View();
            }
        }

    }
}

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