Mars' orbit is not circular, and spacecraft's would not be, either. They would be in slightly different (and intersecting, or nearly so) elliptical orbits, which would allow Mars to gravitationally capture the spacecraft when it approached closely enough.
Is performing a second burn to circularize a craft's orbit to near-sync with Martian orbit really cheaper than a deceleration burn? In fact, that would just be a complete Hohmann transfer.
I'd guess the target orbit for a craft performing this maneuver would still have a periapsis in the neighborhood of Earth's orbit, meaning it'd still be very unlike Mars' orbit until the craft was captured by Martian gravity.
(googles a bit)
It looks like these exploit Lagrange points somehow[1]. Precision maneuvers and weird, long routes to the target body. I'm not quite following how this works with only two bodies (Sun and Mars), versus the Sun-Earth-Moon trio. Wikipedia claims the Mars Orbiter Mission used a low-energy transfer at some point, but I can't figure out whether that was for the insertion into Martian orbit or some earlier maneuver it performed.
