The earliest depictions of the crucifixion show Christ upon the cross, with body erect and with arms stretched straight out. He is clothed either in a long robe or with a loin cloth. There is no attempt to be realistic or to emphasize suffering or agony.
The modern "Christus Rex" crucifix is in the same tradition. Christ stands erect in front of a cross, with arms straight out. The body may be clothed in modern, western eucharistic vestments, and there may be a crown on the head.
The image portrays several concepts at the same time: the historic event of the crucifixion, Christ as the King in his kingdom, and Christ as the victorious sacrifice in the eucharistic feast.