How do you replicate what --private is doing?

[pjhfggij]

I'm struggling to find documentation of what --private option is doing. sandbox.c file shows that it supposed to just put tmpfs over user's home directory, but when i run
firejail --tmpfs=/home/user firefox

it doesn't run, while it does with --private so clearly there is more to it.
Is it documented anywhere? Is it possible to exactly replicate --private using --tmpfs and other options?