A resignation made under duress is invalid under Canon Law. Benedict was under a death threat and the SWIFT system had been shut down in Vatican City as an extra lever to exert pressure. The day after Benedict made an announcement about resignation the SWIFT system for the Vatican was turned back on. There is more than this but there is significant evidence that neither did Benedict resign of unencumbered free will nor did he intend to fully resign the office of the Vicar of Christ but that he intended to "enlarge the office" to be a two-person role having an active and a contemplative/passive member which isn't possible.