Some more memories of my trial of this system, over thirty years ago.
I can't remember where I read about it. Either a Descent article, or a page in the Caving Supplies catalogue. Possibly both? The idea was that it extended the Frog system, with a few extra components and you could flip from Frog to rope walking and back as you climbed. You could rope walk for speed, or use frog to pass rebelays, or other tricky bits.
The loop of the foot loop was tied at such a length that when the top jammer krab was clipped in to the loop itself, the jammer was at knee height. When you changed back to frog, you moved the jammer krab back to the top end knot of the foot loop.
The 4mm or so diameter bungee was clipped, or tied to the back of the harness, came over one shoulder, then was clipped to the top of the hand jammer, an old style Petzl Basic. When in frog mode, the bungee had no tension on it, so could be left in place, or unclipped. When in rope walker mode, it towed the hand jammer up the rope when you stood up on the foot jammer.
The foot jammer was another Petzl basic. I can't remember the details of how it was attached to the foot, but it ended up somewhat higher on the boot than the ankle height of a modern Pantin. I think it may have used something like a 25mm tape waist belt to tie it to the welly boot.
Disadvantages I found:
- There was a risk of the bungeee either getting caught in the Croll jammer, or possibly disconnecting the safety catch and opening the Croll. I did get the bungee caught once. Fortunately, I managed to free it, but there was a good chance of getting badly hung up mid-rope. The Croll design of the time had a safety catch that was easier to accidentally get caught and opened than the modern versions. This never happened to me, but I didn't like the risk. If the croll became disconnected, you could put a potentially high fall factor on the knee mounted Basic's cam teeth, via the dynamic rope safety cord, that could damage, or cut the rope.
- When in Frog mode, the bungee cord was either slack and still attached to the top jammer, or disconnected. There was a risk of it getting caught on cave features in either case and stretched taught, before being noticed.
- Using the old Petzl Basic as a foot jammer wasn't ideal. The strong cam spring made it difficult to get it to run freely, when starting a pitch. The method of attaching to the boot was uncomfortable and tried to twist the ankle. The safety catch meant you had to be sure to be able to reach the catch to release the foot jammer. Not always possible at a constricted rebelay. The modern Pantin was designed to get round all these problems, with a weaker cam spring, proper strap system and initially no (later optional) safety catch.
- Unless you had a very good power to weight ratio, then the extra effort of lifting your weight with one leg, rather than two, got you more knackered more quickly. This was worse when hauling tackle bags, or being knackered. Derigging the upper pitches of a deep and difficult cave of course combines both carrying tackle bags and being knackered!
I found that the extra faff meant that once I'd switched to Frog for some reason, I'd stay in Frog mode. The trial only lasted a few trips and I abandoned it. I only next went beyond basic Frog, when I tried an early Kong foot jammer in around '97. This used one of their Basic knock off jammers, with a better strap system. It had a safety catch. I abandoned it as I couldn't get it to run up the rope reliably. I then started using a Pantin in the early naughties. Recently I found the old Kong foot jammer in the back of the caving cupboard, after losing a Pantin and am using it again after drilling out the safety catch. With this modification, it now works well and I'll probably carry on with it till it wears out. Not PPE, so I'm happy using it.
On a modern version of this knee walking system, a Pantin, or other modern foot jammer gets round the problems I had using an old Basic in this role. You might be able to get round the Croll jamming risk with a different bungee material, like Cavemanmike describes above, or routing. Modern Crolls are less prone to accidental unclipping. The other problems still remain.