https://www.freebsd.org/doc/handbook/adding-swap-space.html

[Dan Hulme]

The handbook incorrectly suggests using

swapon -aq

To immediately enable file-based swap space.  However, without the -L flag,
this does not work as file based swaps are treated as "late" by FreeBSD.
It should read:

swapon -aqL

Thanks,

Dan