What is the use of BackupBkg(...,...,...,...)

Hi,
I found this function while searching for enhancment of bitmaps. Can anyone tell me and give use of the following Win 32 function.

inline void BackupBkg(HDC hdc,HBITMAP hBitmap, int xStart, int yStart)
{
      CoreDrawBitmap(hdc,hBitmap,xStart,yStart,false,SRCCOPY);
}

I kind of like this function . All I expect this function is to take the backgound color temporaryly of the place of the bitmap i.e. push button and thats all.

Hoping to recieve good point
With Best Regards
asser I Qureshi