Around the Medieval Period, the depiction of Christ on the cross would show him with a titled head, usually down towards the right side. It is interesting because it is conveying the very moment of death. Sometimes the tilt is subtle with a gaping mouth, which gives the impression that he taking a final breath.

This is what makes this symbol so powerful. The immanenece of death and in Christian terms, the death of Christ was the payment for our sins, bringing with it a debt of guilt imposes our fate in a way that is hard to ignore. We are not only contemplating the eventuality of death, but the moment of passing, the moment when life ends and death begins, although one could say this symbol reminds us of how every moment is living and dying and breath (the etymological root for spirit) is the indication of this inhaling for exhaltation and exhaling for the fall.

This PDF that @”thetrizzard” shared made me think of this: http://www.unm.edu/~ithomson/Thomson.pdf