The source code

    /**
     * @class Ext.grid.GridDragZone
     * @extends Ext.dd.DragZone
     * <p>A customized implementation of a {@link Ext.dd.DragZone DragZone} which provides default implementations of two of the
     * template methods of DragZone to enable dragging of the selected rows of a GridPanel.</p>
     * <p>A cooperating {@link Ext.dd.DropZone DropZone} must be created who's template method implementations of
     * {@link Ext.dd.DropZone#onNodeEnter onNodeEnter}, {@link Ext.dd.DropZone#onNodeOver onNodeOver},
     * {@link Ext.dd.DropZone#onNodeOut onNodeOut} and {@link Ext.dd.DropZone#onNodeDrop onNodeDrop}</p> are able
     * to process the {@link #getDragData data} which is provided.
     */
    Ext.grid.GridDragZone = function(grid, config){
        this.view = grid.getView();
        Ext.grid.GridDragZone.superclass.constructor.call(this, this.view.mainBody.dom, config);
        this.scroll = false;
        this.grid = grid;
        this.ddel = document.createElement('div');
        this.ddel.className = 'x-grid-dd-wrap';
        // prevent the default action, but don't stop propagation
        this.preventDefault = true;
    };

    Ext.extend(Ext.grid.GridDragZone, Ext.dd.DragZone, {
        ddGroup : "GridDD",

        /**
         * <p>The provided implementation of the getDragData method which collects the data to be dragged from the GridPanel on mousedown.</p>
         * <p>This data is available for processing in the {@link Ext.dd.DropZone#onNodeEnter onNodeEnter}, {@link Ext.dd.DropZone#onNodeOver onNodeOver},
         * {@link Ext.dd.DropZone#onNodeOut onNodeOut} and {@link Ext.dd.DropZone#onNodeDrop onNodeDrop} methods of a cooperating {@link Ext.dd.DropZone DropZone}.</p>
         * <p>The data object contains the following properties:<ul>
         * <li><b>grid</b> : Ext.Grid.GridPanel<div class="sub-desc">The GridPanel from which the data is being dragged.</div></li>
         * <li><b>ddel</b> : htmlElement<div class="sub-desc">An htmlElement which provides the "picture" of the data being dragged.</div></li>
         * <li><b>rowIndex</b> : Number<div class="sub-desc">The index of the row which receieved the mousedown gesture which triggered the drag.</div></li>
         * <li><b>selections</b> : Array<div class="sub-desc">Array of the selected Records which are being dragged from the GridPanel.
         * Unless a CellSelectionModel is being used and the grid is configured <code>dragCell: true</code>, in which case, this will be
         * an Array containing the single selected cell data as <code>[rowIndex, cellIndex]</code>.</div></li>
         * </ul></p>
         */
        getDragData : function(e){
            var t = Ext.lib.Event.getTarget(e),
                sm,
                rowIndex = this.view.findRowIndex(t),
                cellIndex,
                selectedCell,
                selection;

            if (rowIndex !== false){
                sm = this.grid.selModel;

                // Handle mousedown on unselected items (depending on what kind of selection we are using)
                // Select the mousedowned item
                if (sm.getSelectedCell) {
                    cellIndex = this.view.findCellIndex(t);
                    selectedCell = sm.getSelectedCell();
                    if (!selectedCell || selectedCell[0] !== rowIndex || selectedCell[1] !== cellIndex) {
                        sm.handleMouseDown(this.grid, rowIndex, cellIndex, e);
                    }
                    if (this.grid.dragCell) {
                        // Selection is the cell coordinates
                        selection = sm.getSelectedCell();
                        if (!this.grid.hasOwnProperty('ddText')) {
                            this.grid.ddText = '{0} selected cell{1}';
                        }
                    } else {
                        // Selection is the mousedowned row
                        selection = [this.grid.store.getAt(rowIndex)];
                    }
                } else {
                    if(!sm.isSelected(rowIndex) || e.hasModifier()){
                        sm.handleMouseDown(this.grid, rowIndex, e);
                    }
                    selection = sm.getSelections();
                }
                return {grid: this.grid, ddel: this.ddel, rowIndex: rowIndex, selections: selection};
            }
            return false;
        },

        /**
         * <p>The provided implementation of the onInitDrag method. Sets the <tt>innerHTML</tt> of the drag proxy which provides the "picture"
         * of the data being dragged.</p>
         * <p>The <tt>innerHTML</tt> data is found by calling the owning GridPanel's {@link Ext.grid.GridPanel#getDragDropText getDragDropText}.</p>
         */
        onInitDrag : function(e){
            var data = this.dragData;
            this.ddel.innerHTML = this.grid.getDragDropText();
            this.proxy.update(this.ddel);
            // fire start drag?
        },

        /**
         * An empty immplementation. Implement this to provide behaviour after a repair of an invalid drop. An implementation might highlight
         * the selected rows to show that they have not been dragged.
         */
        afterRepair : function(){
            this.dragging = false;
        },

        /**
         * <p>An empty implementation. Implement this to provide coordinates for the drag proxy to slide back to after an invalid drop.</p>
         * <p>Called before a repair of an invalid drop to get the XY to animate to.</p>
         * @param {EventObject} e The mouse up event
         * @return {Array} The xy location (e.g. [100, 200])
         */
        getRepairXY : function(e, data){
            return false;
        },

        onEndDrag : function(data, e){
            // fire end drag?
        },

        onValidDrop : function(dd, e, id){
            // fire drag drop?
            this.hideProxy();
        },

        beforeInvalidDrop : function(e, id){

        }
    });