So basically what I'm wondering is if they are purely an eccentric exercise in terms of hamstrings... if I'm not mistaken, it is the glutes which handle the concentric part?
I got diagnosed with tendonitis in my left lower hamstring a few weeks ago by a physiotherapist. He said for me to start doing Nordic Hamstring curls which have been pretty effective and I can feel my hamstring improving (though still pretty far off 100%).
I'm thinking of including some RDLs into my routine so I can do a little more work on my hams. If this exercise only works the hamstring eccentrically then I'll probably give them a go and just keep it light / high reps for the time being.
Bear in mind I'm referring to the lower hamstring (if that makes any difference)... just the vast majority of what I read online seems to be related to the upper hamstrings.