Here is my understanding on this. If you switch to unsafe side safestrap will restore the backup here and it also says that it backups the safe side but then if you switch back again from unsafe to safe it says that it restores this time the safe but when you boot you get blank screen so you have either to restore a nandroid of the safe side or reinstall the rom. Basically the only backup done by safestrap automatically is for the unsafe side. Correct me if I'm wrong.
Partially correct. Here's how safestrap works from the first install.
You install the apk and the recovery, you boot into the recovery.
You enable safe system, as soon as you do this, safestrap makes sort of a nand backup for the original system. You can then format /system, wipe data/factory reset, wipe cache, wipe dalvik, flash a custom ROM.
When you disable safe system, it makes a sort of nand backup for whichever custom ROM you flashed on the safe system.