It does seem that the sense of your head filling up is because you have only blocked your jugular veins, which should drain the head, and not the carotid arteries. I have done that during my practises. My face also goes dark red.
I think that I can now locate my carotids. Apparently they require more pressure to block. Although it is not painful to squeeze them I find it uncomfortable and unpleasant.
I have not yet blacked out, but I find my sense of hearing alters, surrounding noise gets quieter, then comes back as I release the squeeze.
I am really not looking forward to what I know I must do.