OK, watched the video. He's not doing anything amazing. Hearts just accepts certain values from the registry. When the developers at Microsoft wrote Hearts, they also included some hidden possible values that the program would recognise if there, and that would allow some key combination or something like that. A bit of an Easter Egg, like the flying game in Excel 97.
This guy on the video has just found what this hidden value is on some forum somewhere, typed it in, and it's worked. He hasn't messed around with any C++ or anything more clever like that.