You can open the preview window by pressing p.

Preview window displays output of the jj show (or jj op show) command of the selected revision/file/operation.

You can specify commands to display the content of different item types in the preview window using the configuration options under the preview table in your configuration file.

The default commands are:

[preview]
revision_command = ["show", "--color", "always", "-r", "$change_id"]
oplog_command = ["op", "show", "$operation_id", "--color", "always"]
file_command = ["diff", "--color", "always", "-r", "$change_id", "$file"]

While the preview window is showing, you can press; ctrl+n to scroll one line down, ctrl+p to scroll one line up, ctrl+d to scroll half page down, ctrl+u to scroll half page up.

By default preview window has the following configuration:

[keys.preview]
  mode = ["p"]
  scroll_up = ["ctrl+p"]
  scroll_down = ["ctrl+n"]
  half_page_down = ["ctrl+d"]
  half_page_up = ["ctrl+u"]
  expand = ["ctrl+h"]
  shrink = ["ctrl+l"]
[preview]
  show_at_start = false
  width_percentage = 50.0
  width_increment_percentage = 5.0
  revision_command = ["show", "--color", "always", "-r", "$change_id"]
  oplog_command = ["op", "show", "$operation_id", "--color", "always"]
  file_command = ["diff", "--color", "always", "-r", "$change_id", "$file"]

Pressing P will move the preview window to the bottom of the screen, and pressing it again will move it back to the side. You can configure jjui to show the preview window at the bottom by default by setting:

[preview]
show_at_bottom = true