Back-off:To unscrew drillstring components downhole.
The drillstring, including drillpipe and the bottomhole assembly, are coupled by various threadforms known as connections, or tool joints.
Often when a drillstring becomes stuck it is necessary to “back off” the string as deep as possible to recover as much of the string as possible.
To facilitate the fishing or recovery operation, the backoff is usually accomplished by applying reverse torque and detonating an explosive charge inside a selected threaded connection.
The force of the explosion enlarges the female (outer) thread enough that the threaded connection unscrews instantly.
Backing off can also occur unintentionally.