Activate select to make a selection of voxels that can be copy/pasted, moved, turned, or extruded.
Click and drag the mouse on voxels to define an initial selection. You can then further modify the selection box or use another tool.
Then you can extend a face of the selection by clicking and dragging an edge.
In addition, you can also move the selection by clicking and dragging a face.
The last way to modify which voxels are selected is by clicking and dragging a vertex over the voxels.