Lat Pull Downs
To excel at the vertical forearm freestyle stroke you really need to have big strong lats. And you have to be focusing your stroke so you actually use your lats (instead of your shoulder muscles) to power yourself through the water.
Most weight trainers will focus on building their lats by using a lat pull down machine in the gym. And they will work the lats while seated. I think this is fine. However, some lat building should be done in a standing position and trying to immulate the butterfly stroke whereby the forearms are made to go parallel to the ground first before the lat muscles are engaged in the pull down. This involves pulling the pull down bar toward your face and getting your upper arms and elbows in line with your chest and shoulders.