How to use the @jupyterlab/notebook.NotebookActions.selectBelow function in @jupyterlab/notebook

it('should preserve the mode', () => {
        widget.activeCellIndex = 2;
        NotebookActions.selectBelow(widget);
        expect(widget.mode).to.equal('command');
        widget.mode = 'edit';
        NotebookActions.selectBelow(widget);
        expect(widget.mode).to.equal('edit');
      });

it('should select the cell below the active cell', () => {
        NotebookActions.selectBelow(widget);
        expect(widget.activeCellIndex).to.equal(1);
      });

    execute: () => NotebookActions.selectBelow(nbWidget.content)
  });

execute: args => {
      const current = getCurrent(args);

      if (current) {
        return NotebookActions.selectBelow(current.content);
      }
    },
    isEnabled

    execute: () => NotebookActions.selectBelow(nbWidget.content)
  });

    execute: () => NotebookActions.selectBelow(nbWidget.content)
  });

execute: args => {
      const current = getCurrent(args);

      if (current) {
        return NotebookActions.selectBelow(current.notebook);
      }
    },
    isEnabled

execute: args => {
      const current = getCurrent(args);

      if (current) {
        return NotebookActions.selectBelow(current.notebook);
      }
    },
    isEnabled

execute: args => {
                const current = getCurrent(args);

                if (current) {
                    const { content } = current;
                    if (content.activeCell !== null &&
                        content.activeCell.model.type === 'markdown') {
                        (current.content.activeCell as MarkdownCell).rendered = true;
                    }
                    return NotebookActions.selectBelow(current.content);
                }
            },
            isEnabled

    execute: () => NotebookActions.selectBelow(nbWidget.notebook)
  });