Patrice
No longer a newbie, moving up!
YONGNUO TTL FLASH SPEEDLIGHT YN468-II FOR NIKON
Reading this it seems that S2 is similar to S1 but ignores the pre-flash from the camera's popup and is triggered only by the main flash burst from the popup. In S1 mode the pre flash would trigger the unit. S2 is not a iTTL mode in that the camera will not control the flash duration of this unit. You'll have to set it in TTL mode for that to happen. Now weather TTL will work off camera is not clear, or maybe it is, I did not delve that deeply into the specifics of that flash.
Reading this it seems that S2 is similar to S1 but ignores the pre-flash from the camera's popup and is triggered only by the main flash burst from the popup. In S1 mode the pre flash would trigger the unit. S2 is not a iTTL mode in that the camera will not control the flash duration of this unit. You'll have to set it in TTL mode for that to happen. Now weather TTL will work off camera is not clear, or maybe it is, I did not delve that deeply into the specifics of that flash.