SafeLogin is to ensure that the player cannot be harmed when they are logging in.
It accomplishes this by setting the player to be invulnerable, invisible and flying (to prevent fluid pushing you, sinking into fluid or when you join mid air) on the server side and when pushed by a mob AGGRESIVELY teleports the player back to the center of the spawn block to make them unpushable (in the case of MOVE mode).
If you do step off the block you spawned in on (in the case of MOVE mode), enter the specified command (COMMAND mode), or exit the GUI (GUI mode), the server will remove your invulnerability, invisibility, unpushability, and flying.
