you mean like a PC muscle squeeze? that can help with erection quality and possibly penis length or girth gains, but I dont know about making your balls bigger.
to do a pc muscle flex, next time youre taking a piss, you sort of "flex" your penis muscle (you sort of feel it in your ass) to stop the stream of piss.
theyre called kegels if you wanna do more research on them, but Im not entirely sure thats what you mean by "flexing"