Stack
Stack allows you to copy blocks and entities in a row or grid pattern.
Creating Selections
To create a selection, start by using left-click to select your first corner and then right-click to select the second. You can use middle-click to select faces instead of the corners. This makes it much easier to select blocks that don't already have edges.
See below for a demonstration. The initial selection is made with a Left & Right click, which is then further expanded to include the whole tree using three middle-clicks.
Stacking
Once you're happy with your selection, you can stack it by scrolling with the mouse wheel. Now, a hologram will represent the stack you'll be working with. You will also be presented with different controls.
Using the scroll wheel, you can nudge the selection by one step. Scrolling up will stack the selection away from the player and scrolling down will bring the selection closer to the player, so the direction the player is facing determines the direction the selection is stacked. The arrows on the faces of your selection will indicate the stack direction.
By holding the 'X', 'Y' or 'Z' keys, you can force the selection to stack in a single axis. For example, holding the 'X' key while nudging the selection will force it to stack on the X axis only.
Confirming Selections
After you're happy with your movement and modifications, use right-click to confirm the selection, placing the blocks shown in your selction.
If you're not happy with the result, use Ctrl + Z
to undo the placement. If you can't make up your mind, you can use Ctrl + Y
to redo your undo.
The video below demonstrates how stack can be used on multiple axes.