The General

As long as the General isn't fleeing, every friendly unit within 12" may use the General's own Leadership instead of its own for tests (so it's worth rallying your General first), unless a specific rule requires a unit's unmodified Leadership instead. If the General has a Line of Sight value of 5 or more, Inspiring Presence's range extends by a further 6". Any Leadership modifier a unit would normally apply still applies even when using the General's value instead.

A Steadfast unit may use the General's Leadership for its Break test if it's higher than the unit's own Leadership after combat resolution modifiers are applied -- otherwise it uses its own. For example, a Steadfast unit with Leadership 7 that lost the combat by 1, within Inspiring Presence range of a Leadership 9 General, tests on Leadership 8 (borrowing the General's value). Had it instead lost by 3, it would test on its own Leadership 7, since that beats the General's Leadership 9 reduced to 6 by the same modifier.