The Salisbury Doctrine, or "Convention" as it is sometimes called, emerged from the working arrangements reached during the Labour Government of 1945-51, when the fifth Marquess of Salisbury was the Leader of the Conservative Opposition in the Lords. The Convention ensures that major Government Bills can get through the Lords when the Government of the day has no majority in the Lords. In practice, it means that the Lords does not try to vote down at second or third reading, a Government Bill mentioned in an election manifesto.
Further information on this subject can be found from the following link.
The Salisbury Doctrine (House of Lords Library Note) ( PDF 271 KB)