Draft is overtime. Someone phones you up and asks if you want to work extra day/s for extra cash.
Forced draft is overtime but with no element of choice. Meant to be used only in exceptional circumstances. BA calls pilot, says you're working tomorrow for x days, no choice at all. You are obliged to do the work, irrespective of any arrangements you have made.
Airbus and 777 particularly vulnerable. Obviously if BA can't get hold of you, they can't force you to work, and you get to enjoy the prearranged Christmas/birthday/weekend etc...
Pilots have become very good at avoiding the dreaded call and so BA has been left cancelling a number of flights.