What does it do exactly? What's the difference between SSL on or off during transfer? Thanks in advance.
It allows for faster account transfers at the expense of security.
Basically it disables the encryption between your servers so that you can accelerate the transfers between your old and new servers.
If you are not as concerned about the slight possibility that someone may possibly intercept your client data during account transfers, you can use this option to make the transfers go faster.
Personally, I prefer to leave SSL on but then I do focus a bit more on security.