It fills with water and slows the boat enough to keep jigs near bottom. It's a fancy net-like version of a bucket. Have plenty of rope to adjust the distance of the bag from the boat, and keep a knife handy to cut it free on rough days if it catches on something and threatens to swamp the boat. Attaching the bag to the bow on windy days is much more comfortable, safe, and controllable. Active and passive drifting are also effective on calm days.
Many rivers flow at an ideal velocity for drift presentations then.
Swimming jigs might be rendered inactive when drift speed matches current flow. Active drifting on calm days with a swimming jig can be achieved by using an electric motor to zig-zag a path downstream. Some of the largest walleye I've taken, however, have hit at high noon when I was using only current for drifting. Targeting a zone and tracking information throughout an active drift is key to establishing patterns. I tend to target high-percentage locations close to a weedline, current edge, dropoff, or linear structure, such as a long, narrow shoal.