We don't "grab" the rod with the free hand. The grip is open, very light pressure of the fingers is used to control the direction of the tip of the rod. It may be hard to tell in the video, but the fingers are used by just lightly touching the rod with the fingers left open/loose (not clenched down on the rod). I learned this technique directly from Dr. Ishigaki as the proper method to control hard fighting fish. He cautioned me that this is a very advanced tenkara technique. After spending a week fishing with Eiji Yamakawa, Kyoshi Ishimura, Masami Tanaka, Dr. Hisao Ishigaki, Daniel Galhardo, and Masaki Nakano, I witnessed all of these tenkara anglers use this technique. Must be something to it or they wouldn't do it. That group of tenkara masters and anglers have pretty close to 130 years of combined tenkara experience.
That being said, I have had clients on guide trips break tenkara rods by hard gripping the rod with both hands. My 6 year old son broke his 11' Iwana on a 20 inch rainbow trout by reaching up and grabbing the rod above the grip with his free hand. He did however, dive on the broken rod sections being dragged off by the fish and continue to fight until the tippet broke off.
Just use caution if you find yourself in the situation where you need to use the other hand to help control the tip of the rod. Again, it is a very light touch not used to muscle the fish but to keep the tip of the rod in control. Also the rod Erik is using is 17' long. You need 2 hands to keep that much leverage under control and it takes 2 hands just to cast that rod.
P1010876-2.jpg Masaki Nakano 3rd generation tenkara angler in his family. 20+ years of tenkara experience.