Emacs 26.x: org mode: C-c C-r

[J191]

Having a hard time figuring out what C-c C-r does `context reveal`

Manual says "With a prefix argument show, on each level, all sibling headings. 
With a double prefix argument, also show the entire subtree of the parent."

 - When all the Headings are collapsed and I do C-c C-r
   - nothing happens
 - If I try it on a subheading when it is collapsed
   - nothing happens
 - If I try C-u 2 C-c C-r on a second level heading
   - I hope to see all Heading 2 siblings with the current heading fully 
expanded ?

Is there a setting or something to make this thing work ?
Or does this work only in `agenda views` or `sparse trees` only ?