что происходит при нажатии Enter, когда “главная” команда выполняется

Посмотрите 4. Интерактивный раздел Commands top страница руководства (онлайн или путем ввода man top) для полного списка горячих клавиш.

Вот те извлеченные, о котором Вы конкретно спросили:

 <Enter> or <Space>  :Refresh-Display
      These  commands  awaken  top  and following receipt of any input the entire display
      will be repainted.  They also force an update of any  hotplugged  cpu  or  physical
      memory changes.

      Use either of these keys if you have a large delay interval and wish to see current
      status,

  ? | h  :Help
      There are two help levels available.  The first will provide a reminder of all  the
      basic interactive commands.  If top is secured, that screen will be abbreviated.

      Typing  `h'  or `?' on that help screen will take you to help for those interactive
      commands applicable to alternate-display mode.

  g  :Choose-Another-Window/Field-Group
      You  will be prompted to enter a number between 1 and 4 designating the field group
      which should be made the `current' window.  You will  soon  grow  comfortable  with
      these 4 windows, especially after experimenting with alternate-display mode.

  f | F  :Fields-Management
      These keys display a  separate  screen  where  you  can  change  which  fields  are
      displayed,  their  order  and  also  designate  the  sort  field.   For  additional
      information on these interactive commands see topic 3b. MANAGING Fields.