What is the British practice to capture U. S. sailors and force them to perform tasks in British ships?

This practice is known as IMPRESSMENT/ PRESS-GANG.

Explanation: Impressment can be defined as "the act of seizing for public use".

This was practiced by the British and they used it to capture U.S. sailors into a military or naval force by compulsion, with or without notice.

It was a practice that directly affected the U.S. and was even one of the causes of the War of 1812.