Apple - How to switch or close the new split Terminal pane?

The idea behind splitting is that it allows you to keep a certain part of the shell buffer displayed while continuing to enter new commands. So only the lowest split does allow keyboard input. To position the view on the shell buffer use the scroll bar.

You can un-split by pressing Shift-Cmd-D.

Like the top answer says, to undo the split is ⌘ Shift ⌘Command D.

I just wanted to add that in general OS-X shortcuts that are done with ⌘ Command-char are undone (or the opposite/complement command is executed) with ⌘ Command ⌘ Shift-char.