scroll page then at certian position scroll element

Question

I am at a bit of a loss for how how to do this.

I have a web page. I want a user to scroll down, then at a specific distance from the top I want the mouse scroll to effect an elements position (making it appear as thought that element is scrolling). then when that element hits a position (ie top: -500) I want the scroll to apply to the main webpage again. Any thoughts on how I can do this?

Im working on a fiddle now but not having any luck, I will post when I have something to show

Edit: The beginning of a solution/sudo code https://jsfiddle.net/vk0jk37v/23/

Attached is an image of one area in which I am applying this.

//pageFeature.style.backgroundPosition = "0px " + parseInt(-y / 6) + 'px'); 

var element = document.getElementById('container').getBoundingClientRect();
var elementTop = element.top //distance from top 720;

// variable to stop function from being replayed on scroll when scrolling image
var isScrollingImage = false;

// disables scroll on body
var disableScroll = function() {
    document.body.style.overflow='hidden';
}
// enables scroll on body
var enableScroll = function() {
    document.body.style.overflow='auto';
}
//change position of background along y axis with scroll
var scrollImage = function() {
    console.log("called");
   isScrollingImage = true;
   var pageFeature = document.getElementById("inner");
   var pageFeaturePosition;
   pageFeature.style.backgroundPositionY=parseInt(-scrollY / 10) + "px";
    //if (background is scrolled to bottom) {
    //    enableScroll();
    // }
}

//when element gets to center of viewport and animation is scroll is not on element
//call scrollImage()
function checkPosition() {
    if (scrollY > 720 && !isScrollingImage) {
        disableScroll();
        scrollImage();
    }
    //isScrollingImage = false;
}

//once out of view port will have to bring the image back down, 
//scroll image will only happen on the way down

document.addEventListener('scroll', checkPosition);

Answer 1

    var thresholdY = 150; 
    var thresholdCounter = 0; 
    var speed = 4; 
    var element = document.getElementById('yourElement'); 

    window.onscroll = function(event) { 
      if(scrollY > thresholdY) { 
        window.scrollTo(0, thresholdY); 
        element.setAttribute('style', 'transform:translate3d(0,' + (--thresholdCounter * speed) + 'px,0);'); 
        return false; 
      }
      return null; 
    }; 

Here's a demo: https://jsfiddle.net/pkt6xnb5/

The idea here is that you keep the window in place when the user reaches a certain threshold Y position. Also at that time we transform the element along it's Y axis in the opposite direction the user is scrolling to make it move up as they scroll further down. Is that what you're looking for?