A Flail Weapon is just as it sounds, a weapon attached to a freely rotating component, similar to a medieval flail. They have proven to be very destructive, especially in DSL. Some common examples of bots with flail weapons are Grog, the Vengeful, Iron Spaghetti, and Steel Meatball. As you can see, Flail Shell Spinners are a common use of flails in DSL. Flail weapons tend to brush/scrape the opponent instead of outright smashing them, and can do rapid amounts of damage at a constant rate. Probably the biggest flaw of flailing weapons are their tenancy to havok when ripped off, and in DSL, flails on HS tend to turn the robot, forcing you to constantly fight the motor torque to drive properly.
In Stock, flail weapons are seen most commonly in VSs, HSs and some SnSs. One issue with flail VS is that the flails tend to glitch out of their original position, making for a very wobbly robot. The only way to make a flail in stock is to use a 360 axle mount. These allow for any component attached to them to rotate freely. This can be a nuisance when making horizontal flails.
In DSL, flail weapons are much more common. This is probably due to the DSL axle mounts, which are easier to attach and allow you to control the maximum number of degrees the axle can turn. Quite a few designs use flails, such as Flail Shell Spinners, Flail Hammers and various HS, VS, and SnS. Weapons with high damage potential such as razor tips are ideal for flails.