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I found this article by petzl here. I have trouble following the comparion of the rappel setups with the pull cord. I dont understand how the second one gains this stated advantage:
The next rappel can be set up without threading the rope through the ring.
Would be nice if someone could explain why we gain this advantage. And other techniques that make pull cord rappelling on long rappels efficient.
As noah pointed out, this is very likely an error as the rope has always to be threaded through the ring to end up at the situation displayed. (Assuming you cannot rappel on the pull cord, which is typically the case. Even if the pull cord is strong enough, the diameter difference makes it impossible to service both the rope and the pull cord with the same device) Please note that, with a single strand, friction is a lot lower than on a two-strand rappel and always back up your system.
If you have to rappel a lot, there is no way around double ropes. Twin ropes (always clipping both strands) had their niche for that, but given how thin and light half ropes have become, they are preferable due to the added flexibility of clipping only a single strand to reduce rope drag on the climb up.
edit:
I would not recommend the technique suggested by Benedikt Bauer. While this can be used, it has a high risk of dropping the rope or the pull cord. You need to secure both to mitigate the risk
edit2:
to go into further details on the sense and nonsense of single rope rapelling... the lightest single rope from Petzl is the 9mm Volta at 54g/m. They sell the PUR as a pull cord at 20g/m, giving you a total of 74g/m. The Paso 7.7mm half-rope weighs 40g/m, giving you a total of 80g/m or a difference of 360g for the total package at 60m. Unless you are projecting some sport multi-pitch at your very limit, the easier use during rappel, lower rope-drag during climbing and higher safety against rockfall, etc. of using half-ropes will outweigh the higher weight.
With the second technique, since you don't join the rope and the retrieve cord via a joining knot but just tie a sling knot into the end of the rope and the retrieve cord and clip both into a carabiner. The whole setup can therefore easily be disassembled by unclipping the rope and opening the knot in the rope. Hence, you can after every pitch, when you have retrieved your rope, untie the stop knot, pull the rope's end through the ring, make a new stop knot and clip everything together into the carabiner.
However, as mentioned in the comments: this method will leave you with an unsecured rope while rebuilding your setup. Therefore it should be advised to secure the rope otherwise while opening the knot and pulling the rope end through the next ring.
The first setup with the join knot on the other hand is quite surely too cumbersome to untie and rebuild after every pitch so you will have to pull the whole rope length through the ring at every stand.
