For those not in-the-know, RDL is an acronym for Romanian Deadlift.
This lift should be treated as an accessory movement for the hamstrings & posterior chain. To get the max benefit from this lift, you want the target muscles to be doing the majority of the work (obviously). This is not a test of moving the maximum poundage at the expense of good technique. Use the heaviest weight possible that you can handle with PROPER FORM.
Here's an example video along with a few cue's to help you not look like a dumb-ass at the gym; (Note: EXCESSIVE VOLUME WARNING)
-Initiate the movement by pushing the hips backward
-Maintain a SLIGHT bend at the knee
-Do not lower the bar further than you are able, while maintaining a strong (flat) thoracic & lumbar spine position
-Bar should glide along the thigh/shin, or very close to it
-Raise & Lower the bar in a smooth fashion, concentrate on keeping tension on the target muscles
In my (completely non-professional) opinion, this lift is best done for medium to high repetitions & total volume. For example; 4-5 sets of 6-12 reps. If done correctly you WILL feel tremendous tension on the hamstrings and glutes. As well as crippling soreness the following day or two.