I've been doing S&H for the last 4 weeks, so I know exactly which exercise you mean. It is the rear delts raise. I have to say that the way this exercise is done in S&H is the best, it's the only time I ever really feel the rear delts working. It's not a tricep move, but since you are lifting with the back of the shoulder, the back of the arm will come into play also, but only slightly. I think this is inevitable.
However, what makes this move work for me is the retration of the shoulders, keeping them up and back, just as you do with dumbell rows for the back. I just rushed downstairs to grab my dumbells and perform this to see to what degree the triceps get involved and it is very little. I tried it again with poor form, i.e. letting my sshoulders just hang down, and then, yes, it became largely a tricep move and I could not feel the shoulders working. In fact. it felt most uncomfortable.
When I do these rear delt raises, I check that my shoulders are retracted before the ascent of the weights again with every rep, because it is easy for them to fall out of the correct alignment and start to just hang. It's a tiny, minor and quick adjustment and you just slip it in before raising the weights again. I think this may be what is happening to you, and it is causing you to call on the triceps instead of the rear delts. Try doing this liuttle adjustment with every rep.
Do you understand what "retration of the shoulders" means? Really, you are lifting your shoulders up and then pulling your shoulder blades a little together, locking them into place. It feels very secure once you have it. Practice just retracting the shoulders, many times, before even picking up weights. Deliberately let your shoulders hang, so you can feel the incorrect position, then keeping your arms completely still, just pull the shoulders up within their sockets. When I do this I feel the muscles in my back contracted also.
Good luck,
Clare
