Heavy back training without straps will provide a lot of stimulus for the forearms, but it never hurts to do a little extra at the end of back, or on a seperate day.
My favourites are:-
* Behind the back forearm curls with a barbell, letting the bar roll down as far as possible before bringing it back up.
* Reverse forearm curls superset with forearm curls, using an EZ bar, builds an incredible pump.
* Reverse barbell curls, superset with a pair of grippers.
Always go to failure as well, so that your forearms are so pumped up, they feel like stone!!!