Here's Daniel Scher's review which sums up the 'Auto Complete' feature:
A Nice Twist on Flow
This game could really use a clean explanation of its rules, so here goes: Connect identical symbols in the grid by drawing paths. Your paths must not cross, and you must traverse every empty grid space. So far, this is identical to Flow. But here's where it gets better: If you can figure out a way to draw as short a path (or multiple paths) as possible so that the game can auto-complete the remaining paths, you'll score higher and receive more stars. When will the game auto complete a path? If it can travel between two identical shapes without ever having to decide which way to turn. So if a path contains an empty 2x2 grid of squares, the game will not recognize this path as one that can be auto completed. Remember, just because you can see the simple way to connect two identical shapes doesn't mean the game can.
I've played the game. I like the Auto Complete feature. But not enough to continue playing. However, I highly respect Daniel's opinion and know that some of you will especially like this version of Flow.