An arc flash or arc flashover occurs when an electrical discharge goes through air. It can cause a bright visible light as well as damage the two objects it lands on.
When a hotline tap clamp is installed directly on a main line the arc goes from the bill of the hotline tap clamp to the main line.
However, when a stirrup clamp is used, the arc goes from the bill of the hotline tap clamp to the bail of the stirrup, thus eliminating damage to the main line.