I have developed lateral epicondylitis and, while shopping for flexbars to train internal and external rotation of the forearm, I came across this one and decided to try it since it was very cheap (€6).
After a few tries, I think it disappointed me, although that was not a surprise.
More specifically, the packaging states that it has variable resistance, which apparently changes by moving the middle connector back and forth along the springs. The only problem is that there is no way to lock it, so it keeps moving no matter how careful you are. Changing hands 3 times to do 3 sets, it moved all 3 times while I was trying to keep it still, getting on my nerves. Then, the movement of the connector itself slightly changes the geometry of the grip, so each time it feels different in the hand.
Also, this device as designed can only train internal rotation, for medial epicondylitis (the grip shown in the photos), but it does not work for external rotation, for treating lateral epicondylitis, which was the main thing I was interested in.
After playing with it for a day, I put it aside; fortunately, it was dirt cheap. Needless to say, I would not buy it again, as it is annoying, awkward for frequent hand changes, and I cannot do external rotation for lateral epicondylitis. It also creaks a lot and the middle connector makes a clank-clank sound as it bangs against the spring during strong flexion, and the cap covering the screw on the end grip popped off on its own within the first 5 minutes and does not stay in place well