Let's say you have a $secureString. If you don't have one handy, create one:
PSH> $secureString = ConvertTo-SecureString -Force -AsPlainText "hello"
Actually, we're breaking the system - SecureStrings aren't supposed to be created this way - they're supposed to be created like this:
PSH> $secureString = Read-Host -AsSecureString
Then again, we're talking about converting it back to text, which is also breaking the system. Or at least circumventing it.
There we go.
Disclaimer: this isn't anything new. Many blog posts have gone over the same stuff. This just presents it as a one-liner, and because I found it useful (primarily just to read a password without displaying it.)