To use you must have installed SBSettings from Cydia Installer. Bring up SBSettings, then select Processes. From here you can delete the task you want to kill.
Only tasks you launched from springboard and may want to kill are included. So you won't be able to kill any tasks that might cause system problems.
Along with the Processes Widget, BigBoss has also released SBSettings v1.0.7 which adds a total available memory line on the text display.