SafeLogin

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 and invisible on the server side and sends the client a packet telling it to make the player un-pushable client side (I tried to make the client un-pushable server side, didn't work :( so both sides is required), so long as they stay on the block they logged in on. So, for instance, if you are in an ocean or logged off in a running water stream, it will disable it as you will be moved off your initial spawn block, may be able to fix it in a future update though.

Moving on, if you do step off the block you spawned in on, the server will remove your invulnerability, invisibility and will send the client 1 last packet to make you pushable again. That is all there is to it.