show only the current depth

i would like to see this as well.

i was able to set this up by editing the block settings to include:

Menu: the menu selected by the page
Starting level: 1st level
Make the starting level follow the active menu item: checked
maximum depth: 1
expand all children of this tree: checked
sort menu tree by the active menu item's trail: checked
fixed parent item: the menu selected by the page

hope this helps.

One issue I find with this solution is that there is no indicator that a list is expandable. So if a visitor to a site sees the menu, they don't know they can drill deeper into "1". This is part of the problem I have (http://drupal.org/node/1221634) in that either the menu is fully exposed or not...there are no visual clues that you can drill deeper into the menu - and there is no way to collapse a section of a menu. It would be great to see:
> 1
o 2
o 3
and then
\/ 1
o 1a
o 1b
o 1c
o 2
o 3
...and know that the menu is expandable. It would be great to have options to expand/collapse the menu and in addition to the "maximum depth of the menu tree", have "maximum *initial display* depth of the menu tree" (set from 1-unlimited)...and show expand/collapse prefixes to the menu items.

If you are talking about showing only the menu items under the active menu items then I would do this:

Get the patch from https://www.drupal.org/node/1620298#comment-6406618
Starting level: 1st level
Make the starting level follow the active menu item: checked
maximum depth: unlimited
expand all children of this tree: checked
fixed parent item: the menu selected by the page
Starting Level: Children of active menu item, and active menu item if no children

Hope that helps