I remember working at GNC when they first came out years and years ago. It basically binds to the fat molecules and prevents them from being absorbed, which means they pass through your system mostly in tact. Long story short, you get the squirts. Sometimes urgently.
If your PB habit is bad, buy and use smaller spoons. (Seriously). "Fat binders" are a shortcut that can't determine between healthy fats or bad fats, so overall, they're not worth it.