code: function _mouseDown(event) {
  
          // Check whether mouse-scrolling is enabled
          if (!this.options.mouseMove) {
              return;
          }
  
          // Reset any previous mouse-move operation that has not yet been
          // cleared.
          if (this._scroll.mouseMove) {
              this.releaseScrollForce(this._scroll.mouseMove.delta);
          }
  
          // Calculate start of move operation
          var current = [event.clientX, event.clientY];
          var time = _getEventTimestamp(event);
          this._scroll.mouseMove = {
              delta: 0,
              start: current,
              current: current,
              prev: current,
              time: time,
              prevTime: time
          };
  
          // Apply scroll force
          this.applyScrollForce(this._scroll.mouseMove.delta);
      }
docs: Called whenever the user presses the mouse button on the scrollview