Supinated means that your thumbs are pointing outwards like in rings turned-out support. To get in tuck back lever start by grabbing the rings, lift yourself all the way up to inverted hang position. From what I hear it's going to be hella important for iron cross, but the process is going to be slow, tendons take a long time to strengthen, so don't rush it.

Try a back lever sort of position with your feet on the ground, bearing some of your weight. Progressions. Shoulder position is slightly protracted or neutral. Remember, you'll need a set of rings and plenty of room to maneuver to pull this off, so only attempt the front lever … Here the back can be slightly relaxed. Keep your head neutral, don't look down towards your feet. Open up … Once you're warmed up, you're ready for the progressions. First one among the back lever progression, this hold can be considered easier one of the lot. Starting in the regular position from the last progression, start to open up out of your ball, straightening your back. Slowly progress by bearing more weight through the arms. Back lever progressions Skin the cat - practicing the skin the cat position will help you build basic strength for the inverted hang. Now bring your knees closer to your chest. Progression 4: Advanced Tuck Back Lever (5 Sets of 12 Seconds Holds) This progression adds more difficulty to the regular tuck back lever. All back lever progressions should be done with a supinated grip.