I'll attempt a simple explaination of the Ekich Bobbin. Others can dive in further on detailing the mechanism of this magnificent tyer's tool.
There is a clutch and spring in the bobbin's housing. There is a pin that engages the thread spool. There is a limit to how much thread can be auto-wound. I do believe that 10 inches is well within that range. You pull the bobbin down away from the vise and it stays just where you put it. Lift a little and it auto-winds and then stays put there. Effortlessly. To take up the slack even further, simple move the bobbin up towards the vise and the thread winds back on the spool-automatically. You feel/hear no catching or ratcheting movements within the bobbin. No need to pull on it to get the bobbin to retract. It is smooth as silk and automatic. I have learned to use the "take-up" action of this bobbin to tie hitches very quickly.
There is a short learning curve when beginning to use the Ekich Bobbin. Some roughly manufactured thread bobbins need a little manicure in order to get them to run smoothly. If there is still stored tension in the bobbin after you are done with the fly, then you need to learn to use the provided finger grip and release the stored enegy by slipping the spool away from the bobbin houseing to release that thread tension. Otherwise, your thread will be retracted completely back onto your spool and you will have to re-thread the bobbin again for the next operation.
I really enjoy using the Ekich bobbins. I find it speeds-up my tying and allows me to be in control better than when using a simple, standard bobbin. It save steps.