Thrales perform dances based on the traditional rapper sword dances of North-East England. The dances were originally performed by the miners from the pit villages of Tyneside.
A rapper dance consists of 5 dancers joined in a ring by flexible metal swords (the rappers). Through a series of high-speed and possibly acrobatic jumps and movements, the dancers tie the swords into a series of knots in various shapes to display to the audience.
The dance is accompanied by traditional folk music and hard soled shoes are worn by the dancers who punctuate the dance using percussive steps.
Many sides, including Thrales, are often joined by characters to announce the dance and interact with the audience. These may include a male "Tommy" character and female "Betty" character, and are often played by someone obvious of the opposite gender.
Although Rapper can often be seen performed outdoors alongside other folk traditions, such as Morris dancing, it's generally more suited to more confined indoor spots. As a result it's often danced inside busy pubs.
More details of the history and origins of the dance can be found at Rapper Online.