Intercept storage in database (onBeforeSaveField? if, how?)

Hi,

First of all I want to thank you for this outstanding product! I would like to know however: how can I intercept data from an Item "during" saving (right before or right after an Item is saved to the database)?

I want to read a particulair field, do something with it in PHP and store it in a table: I've read about the Fieldplugin "onBeforeSaveField" in the feature list but apparently there's no further explanation on this topic (or is there? please be so kind to point me in the right direction in that case).

Could someone please explain a bit more how to do this? (I'm kind off lost here I guess, sorry...).

Appreciate your help!