The source code

    /**
     * @class Ext.grid.CheckboxSelectionModel
     * @extends Ext.grid.RowSelectionModel
     * A custom selection model that renders a column of checkboxes that can be toggled to select or deselect rows.
     * @constructor
     * @param {Object} config The configuration options
     */
    Ext.grid.CheckboxSelectionModel = Ext.extend(Ext.grid.RowSelectionModel, {

        /**
         * @cfg {Boolean} checkOnly <tt>true</tt> if rows can only be selected by clicking on the
         * checkbox column (defaults to <tt>false</tt>).
         */
        /**
         * @cfg {String} header Any valid text or HTML fragment to display in the header cell for the
         * checkbox column.  Defaults to:<pre><code>
         * '&lt;div class="x-grid3-hd-checker">&#38;#160;&lt;/div>'</tt>
         * </code></pre>
         * The default CSS class of <tt>'x-grid3-hd-checker'</tt> displays a checkbox in the header
         * and provides support for automatic check all/none behavior on header click. This string
         * can be replaced by any valid HTML fragment, including a simple text string (e.g.,
         * <tt>'Select Rows'</tt>), but the automatic check all/none behavior will only work if the
         * <tt>'x-grid3-hd-checker'</tt> class is supplied.
         */
        header : '<div class="x-grid3-hd-checker">&#160;</div>',
        /**
         * @cfg {Number} width The default width in pixels of the checkbox column (defaults to <tt>20</tt>).
         */
        width : 20,
        /**
         * @cfg {Boolean} sortable <tt>true</tt> if the checkbox column is sortable (defaults to
         * <tt>false</tt>).
         */
        sortable : false,

        // private
        menuDisabled : true,
        fixed : true,
        hideable: false,
        dataIndex : '',
        id : 'checker',
        isColumn: true, // So that ColumnModel doesn't feed this through the Column constructor

        constructor : function(){
            Ext.grid.CheckboxSelectionModel.superclass.constructor.apply(this, arguments);
            if(this.checkOnly){
                this.handleMouseDown = Ext.emptyFn;
            }
        },

        // private
        initEvents : function(){
            Ext.grid.CheckboxSelectionModel.superclass.initEvents.call(this);
            this.grid.on('render', function(){
                Ext.fly(this.grid.getView().innerHd).on('mousedown', this.onHdMouseDown, this);
            }, this);
        },

        /**
         * @private
         * Process and refire events routed from the GridView's processEvent method.
         */
        processEvent : function(name, e, grid, rowIndex, colIndex){
            if (name == 'mousedown') {
                this.onMouseDown(e, e.getTarget());
                return false;
            } else {
                return Ext.grid.Column.prototype.processEvent.apply(this, arguments);
            }
        },

        // private
        onMouseDown : function(e, t){
            if(e.button === 0 && t.className == 'x-grid3-row-checker'){ // Only fire if left-click
                e.stopEvent();
                var row = e.getTarget('.x-grid3-row');
                if(row){
                    var index = row.rowIndex;
                    if(this.isSelected(index)){
                        this.deselectRow(index);
                    }else{
                        this.selectRow(index, true);
                        this.grid.getView().focusRow(index);
                    }
                }
            }
        },

        // private
        onHdMouseDown : function(e, t) {
            if(t.className == 'x-grid3-hd-checker'){
                e.stopEvent();
                var hd = Ext.fly(t.parentNode);
                var isChecked = hd.hasClass('x-grid3-hd-checker-on');
                if(isChecked){
                    hd.removeClass('x-grid3-hd-checker-on');
                    this.clearSelections();
                }else{
                    hd.addClass('x-grid3-hd-checker-on');
                    this.selectAll();
                }
            }
        },

        // private
        renderer : function(v, p, record){
            return '<div class="x-grid3-row-checker">&#160;</div>';
        },

        onEditorSelect: function(row, lastRow){
            if(lastRow != row && !this.checkOnly){
                this.selectRow(row); // *** highlight newly-selected cell and update selection
            }
        }
    });