code: function () {
  			if (_elems) {
  				var progress = Scene.progress();
  				if (progress != _currentProgress) { // do we even need to update the progress?
  					if (Scene.duration() === 0) {
  						// play the animation
  						if (progress > 0) { // play forward
  							animate(_elems, _properties, _options);
  						} else { // play reverse
  							reverse(_elems, _options);
  							// velocity(_elems, _propertiesReverse, _options);
  							// velocity("reverse");
  						}
  					} else {
  						// TODO: Scrollbound animations not supported yet...
  					}
  					_currentProgress = progress;
  				}
  			}
  		}
docs: Update the tween progress to current position.
  @private