DdRT or doubled rope technique is the footlock competition in the tree climbing competitions. Some people want to make the differentiation from double rope that is used in rock/ice work tha actualy uses 2 separate ropes or DRT.
I use a samll d to infer that the second d is not a word in the abreviation.
Mechanical ascenders or loop hitch, foot locking up two ends of a rope draped over a crotch. climbing line is not attached to the harnes for climbing.
Mikes DdRT picture apears to be the traditional loop used for working limbs.
Now can I add anything else to muddy the waters even more?