When level of your system memory falls below 100 Mb, it starts probing memory usage. By using such probes, Memory Cleaner detects silence interval (when your memory is less used). Then performs very quick memory freeing.
While we think that 100 Mb is good setting for everyone, you can decide to change it. You can change the warning level using Terminal.app.
defaults write com.alice.mac.memorycleaner "Warning Level" -integer 100
Where 100 is a warning level in megabytes. Please change this preference only if you understand what you are doing.