580d.  No judgment shall be rendered for any deficiency upon a note
secured by a deed of trust or mortgage upon real property or an
estate for years therein hereafter executed in any case in which the
real property or estate for years therein has been sold by the
mortgagee or trustee under power of sale contained in the mortgage or
deed of trust.
This section does not apply to any deed of trust, mortgage or
other lien given to secure the payment of bonds or other evidences of
indebtedness authorized or permitted to be issued by the
Commissioner of Corporations, or which is made by a public utility
subject to the Public Utilities Act (Part 1 (commencing with Section
201) of Division 1 of the Public Utilities Code).

Note:
This is frequently combined with CCP 726 "the one action rule" to block senior lenders from collecting a deficiency.
If the lender forecloses via the power of sale clause (not in a judicial proceeding) that lender is waiving its right to 
collect on the deficiency... in most circumstances.  
Please be careful if you have two loans.  The second loan is rarely going to be blocked by this rule.
Sold out juniors not subjected to 580b are likely to be able to go after you for the deficiency.
Your attorney may use this law to gain leverage during your short sale negotiations