Playing with the standard shade (stringy thingy with the back-stabber spools on each end) and found there is no tension device in it. It's just one string that runs from one spool, up-across-down the shade to the other spoolie.
Tension appears to depend on how hard the installer pulls on the string, then they tie a knot and cut off the excess after the spool is mounted on the triangle corner plate.
ETI has that metal triangular plate fabricated to mount the spool. Working up a more flexible version for those windows where we sit.
I ordered all these parts thru ETI to dismember and play with. Wifey said NO to taking apart our 21 shades..........