It will be of no surprise if we tell you that a Suppedaneum Cross is any cross featuring (...wait for it...) a suppedaneum. And a 'suppedaneum' is nothing to do with a 'heroic yet indecisive person from Denmark'. That would be a 'super Dane, ummm....'
The word 'suppedaneum' is a compound of the Latin sub ('under'), + ped ('foot'), + aneum (a noun and adjectival suffix that inventors of languages like to add from time to time). But rather than saying 'subpedaneum', the 'b' was changed to a 'p'. This avoids, what linguists call, a 'voiceless glottal plosive' or 'glottal stop'. In other words, it makes it easier to pronounce.
A suppedaneum is a small platform affixed to a cross for supporting the feet of the crucified. We shouldn't presume that this platform is a mere footrest, any more than the main upright pillar of a cross is the victim's backrest. The suppedaneum, if present, is a brace for the feet to push hard against to prevent the victim from suffocating.
If a person is hung by their wrists, the weight of the body forces the rib cage to expand which makes normal breathing difficult. (Try it now. Push your hands up above your head, as high as you can, and try to breathe normally.) If the victim is weak from previous torture, then such hindrance to breathing can cause suffocation and death. The objective of the crucifixion was to cause a slow and painful death. The suppedaneum therefore was a cruel method to prolong the suffering.
Another purpose was to prevent the body from falling from the cross. Without such a device, if the pillar of the cross was dropped into a hole, the nails could tear through the flesh or the ropes could rip off the limbs.
We do not know for certain whether suppedaneum was present on the cross used to crucify Jesus (although there's a prophetic reference to one in Ps. 99:5). A sedile (small saddle) may have been attached to the front of the cross to serve the same purpose.
The suppedaneum is prominent in Eastern Crosses, especially the Russian Orthodox (Crux Orthodoxa), Byzantine and Slavonic crosses. These can all be considered a variation of the Patriarchal Cross, having two smaller crossbeams, one at the top and one near the bottom, in addition to the longer crossbeam (patibulum).
The suppedaneum on crosses of the Greek Orthodox Church and Ukrainian Catholic Church is invariably horizontal. On other Eastern crosses it is often diagonal, with the side to the viewer's left usually being higher. A traditional interpretation for the slant is that it symbolizes a balance scale showing the good thief St. Dismas, having accepted Christ, would ascend to heaven and be on God's right hand, while the thief who mocked Jesus would descend to hell. In this interpretation, Christ and the Cross is a balance of justice, reminding us of the Final Judgement.
Another explanation of the slant reflects half of the 'X' shaped Saltire cross of St. Andrew, since he is credited with introducing Christianity to those Eastern countries (today's Romania, Moldova, Belarus, Russia, and Ukraine). The story goes that when Andrew preached in southern Russia, he used a large three-bar cross as a visual teaching aid. All three bars were parallel, and when relating the Passion he tilted the lower footrest to signify that those on the right side of Christ will rise up into heaven and those on the left will slide down into hell. (See also Right-hand side of God.)
Whether horizontal or diagonal, the foot-rest of Christ is connected with Old Testament verses about bowing down to the "footstool of God". In Psalm 99:5 we read:
Exalt ye the LORD our God, and worship at his footstool; for he is holy. (KJV)
Note that God's footstool is described in Matthew 5:35 as "the earth" but the Psalm does not say that we should worship the earth, it does not say we should worship a footstool. The Psalm says we should worship at the footstool, i.e., at the feet of Christ. The more we humble ourselves before God, the more we exalt him.