This patch is very simple and allows a person to limit the menu_tree_prune_active_tree() to stop when it reaches a depth of $config['depth']. This way we can display submenus of a part of the website. Take the following main menu tree as example
home
about
services
services/graphicdesign
services/webdesign
services/photography

What I was wanting was a submenu to appear on all the services pages. So with my code changes made I can choose 'depth' of 1 in the configuration for the block and then the menu_tree_prune_active_tree() will only follow the active trail that menu levels deep before returning. This way if any subpage from the services tree is active we will show a submenu starting at services.

I have never made a real patch so I'm just including the few lines of code here.

//in menu_block.module
// Prune the tree to the active menu item.
if ($config['follow']) {
menu_tree_prune_active_tree($tree, $config['follow'], $config['depth']);
}

function menu_tree_prune_active_tree(&$tree, $level, $depth) {
module_load_include('inc', 'menu_block', 'menu_block.follow');
_menu_tree_prune_active_tree($tree, $level, $depth);
}

//in menu_block_follow.inc
function _menu_tree_prune_active_tree(&$tree, $level, $depth) {
$current_depth = 1;
...
...
} while ($found_active_trail && ($current_depth++ < $depth));
}