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As I try to wrap my brain around this could I assume then in order for this to work, on approach to the planet you have to be going in the same direction as its orbit?

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Theoretically no, but if you're going the opposite direction the approach velocity is vastly higher and thus you need a (1) very massive planet and (2) much lower periapsis to have meaningful gravitational slingshot effect. In practice, between atmospheres (the big planets being Jupiter, Saturn, and Neptune), and (for non-gaseous planets) planets not being perfectly smooth spheres, that probably decreases the tolerances below any acceptable level in this solar system.

> decreases the tolerances below any acceptable level in this solar system.

I've heard this referred to as "periapsis below mean radius" which is a euphemism for "the mission just ended."

Yes, generally impact with terrain is unacceptable ;-).

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