Because a YES share and a NO share always add to $1, buying YES is equivalent to selling NO (and vice versa). This strategy quotes both the YES and NO order books together, routing each maker order to whichever book offers a better spread or queue spot while keeping your net position flat. The edge is doubling the places you can provide liquidity and capturing differences between the two complementary books.
Markets on Polymarket where complement-book cross-quoting is the natural play: