For my knowledge I think is not possible to have rear curtain sync on a Canon + Inon.
I think that Canon needs to recognize the flash to allow the rear curtain sync. It does not recognize the Inon as a strobe, so...
I do not have that on my Canon 40D with the Inon. I am waiting for a new housing for a Canon 7D MKII, I has willing to have that with the use of the internal flash + an optical cord, but I am not sure if that is possible.
I do not know if Ikelite strobes with Canon camera on ikelite housing allow you to have rear curtain sync. When I had a system like that, the camera (canon 350D) did not have the rear curtain sync function.
I think that the electronic circuits that allows to activate the strobe with a led are not recognized by Canon cameras as compatible flash, so the camera does not allow the rear curtain sync also.
If someone knows how to have that between a Canon camera and the Inon, I am also interested to learn!
It can be a small grain of sand also. When rinsing, push the button and turn it both sides till it frees the salt or the sand. It is a patience game....
The "thingy" is upfront a sensor for the external auto function of the strobe. If there is too much light and you need to improve the accuracy of the external auto function, you can move the wheel and the area upfront the sensor get narrower and the sensor gets the sensivity reduced by 0.5 to 1 stop.
But it only has impact on the external auto (a function that allows the strove to measure by it self the amount of light that the subject you are litting reflects and calculates the right amount of strobe power. Only for the external auto position of the left dial.
If you do not use external auto, you does not need it, neither it will mess with you.