video of my charging gauge...whats it doing?

aftermarket gauge and 1970 dart 400 msd system 6al. i wish i could upload the video the owner of the car said a shop had added a diode or something to make it charge that way.... i video would be worth a 1000 words right now... lol i do appreciate the input. i am not having any problems from it... the system works fine... ive just never seen a gauge do a pulse like that.. and have no idea what the shop he used has done to make it bounce a rythm on the gauge