[lxmorj]

You can only slingshot in a direction another body is orbiting (to gain velocity). You're harvesting it's momentum, basically!

[anotheryou]

Not sure if the term itself restricts it to an accelerating maneuver, but of course you can do the reverse and slow down.

[lxmorj]

The more I try to read about this the more unsure I am I truly understand it, so perhaps not...