Borane as a reducing agent
Henry Rzepa gives a mechanism in which two borane molecules attach to the carboxyl group of a carboxylic acid.  The boron atoms are attached to the electronegative but electron-rich portions of the carboxyl group (meaning the oxygen atoms) and then transfer hydride ion moieties to the carbon while taking away one of the oxygens.  This exchange of hydrogen for oxygen reduces the carboxyl carbon.  Other carbonyl compounds may be assumed to react similarly.