The Directories->Collapse All Trees menu item collapses all existing directory trees. It is primarily useful in Multiple Trees moding. The trees may be individually re-expanded in Multiple Trees by a LMB single click on the desired drive letter.
This option will also collapse a single directory tree from any subdirectory. The Directories->Collapse All option requires the selected directory to be the root directory to achieve the same result.
Accelerator Keys - Ctrl Shift -