"Chemical shift"

I read some explanations and definitions, still do not have it clear. Any simpler wording from any of you, please? (Nuclear magnetic resonance spectroscopy)

http://www.cis.rit.edu/htbooks/nmr/inside.htm

http://www.scirus.com/srsapp/search?q=%22Chemical+shift%22&ds=jnl&ds=nom&ds=web&g=s&t=all

From what I grabbed; when exposing an element to a magnetic field, its resonant frequency changes (shifts) certain frequency. As for Au, it is 1.712690 MHz. For Ag is 4.653623 MHz, ...

Now,

-That amount of shift corresponds to a fixed reference amount of field, say 7 Tesla. ?

-Is the shift less for a lower intensity magnetic field ?

-What are the Ag, Au resonant frequencies without the shift because of the field, or at its 'rest' condition ? Higher or lower Hz amount ?

-Is there a 'decay time' of the shift after removing the magnetic field or is it instantaneous ?

Miguel