Sounds like it's that thing in KSP where you time your insertion to The Mun's sphere of influence just right so you don't have to burn retro to achieve Munar or

Sounds like it's that thing in KSP where you time your insertion to The Mun's sphere of influence just right so you don't have to burn retro to achieve Munar orbit. Basically hit it so that you do gravity-assisted deceleration, and end up in orbit around it—typically badly elliptical orbit, but that's cheap to correct.