Overshot
Overshot is a fishing tool for catching the fish externally. Most overshots have primarily three parts. Top sub, bowl and guide. Top sub has connection for the running string. Bowl houses packer, spiral grapple and spiral grapple control. Guide directs the tool on top of fish. Packer is a rubber ring.
As the overshot moves down over the fish, the rubber provides a sealing around fish making it possible to pump drilling fluid down the fish and to circulate it under pressure outside of the stuck fish. Bowl has tapered helical spiral on the inside. Grapple has identical tapered helical spiral on the outside.
Grapple control keeps the grapple into its place. The tool is slowly rotated close to top of fish. As the guide washes over the top of the fish and brings it inside the overshot, the top of the fish is swallowed by the grapple. A slow upward pull tends to move the grapple down. The helical wedges on the inside of the bowl make the grapple to contract and grip the fish tightly.