Keep your finger on the pulse

Does anyone know the meaning and origin of the phrase "keep your finger on the pulse"? I checked a bunch of dictionaries but didn't really get the phrase. Even on this site there's a listing of it, but I couldn't find any definition for it. Please help.

If one has one's finger on someone's pulse, one is in a position to monitor the working (to some extent) of their heart. By extension, if one has one's finger on the pulse (of political activity, for example), one is in touch with what is going on.

DFG