Underbrace
This command
starts a "growing" underbrace.
The cursor sets above
the underbrace symbol , so you have just to type the main part.
To finish it, type Enter as usual.
Then the cursor sets below
the underbrace symbol, so you have just to type the under part, for instance n times.
To finish it, and then finish the
symbol, type Enter
as usual.
The keyboard shortcut is Tab f.