Proposed Answer: Under the theology of Christus-Victor, the "Power of Sin" is the Laws, and Traditions, that were decreed against people, because of sin.
Note: Please forgive any inaccurate representations which may be present, as I attempted to extract this answer from Greg Boyd's 2015/05/17 Video Explanation :
In the theology of Christus-Victor, early Christians believed that Christ overcame Sin, its Power, Death, Rulers and Authorities, essentially by allowing these principals to self-implode, publicly exposing and humiliating themselves.
God, in love, determined to set people free from their own slavery to sin, death, and its decrees by sending his son so that death could no longer have mastery over them, (John 3:16).
The Power of Sin:
The Power of Sin are decrees against people, because of sin, (1 Cor. 15:56). Decrees are laws, and traditions, Col. 2:8 & 20-23).
God's Intent, to Disarm:
God's intent was to render the Devil powerless, who held the "Strength of Death", (Heb. 2:14).
How these Authorities and Rulers Humiliated Themselves:
God exposed how utterly "Sinful", "Sin" actually is, (Rom. 7:13), by allowing the "Rulers" and "Authorities" to nail the "Power of Sin" to a tree, (Col. 2:14), (that is the debt of decrees, laws and traditions against people), exposing and humiliating this debt, and these authorities, (Col. 2:15), as cursed, (Gal. 3:13), as they unjustly imputed death to a man who was good, by manipulating the good law of God, (Rom. 7:13).
And, had the "rulers of this age .. understood; .. they would not have crucified the Lord of glory", (1 Cor. 2:7-9);
Greek Note: The "Public Display" of the "Rulers" and "Authorities", (Col. 2:15), is the same Greek Construction, (ἐδειγμάτισεν ἐν παρρησίᾳ), bears the same connotation used to state that Joseph did not want to "Publicly Disgrace" Mary for being pregnant outside of marriage, (αὐτὴν δειγματίσαι, Matt. 1:19).
Sin disgraced itself by crucifying an innocent man, Jesus, who was perfectly obedient, even to death, even death on a cross, (Phil. 2:8). If the law brought death to an innocent man, then how incapable is the law of bringing life to the guilty, (Gal. 3:21)? Moreover, those decrees have no value against fleshly indulgence, (Col. 2:23).
The evidence of the "worthlessness" of this "indebtedness machinery" was publicly exposed, nailed to a cross, proving, once and for all, that there was no life in them, (Gal. 3:21).
Forgiveness, and the Removal of Debt:
By publicly disgracing the debt of decrees against people, (laws, and traditions), God cancelled debt to them, (Col. 2:14)--because there simply was no value in them.
Trusting in This Work of God to Life, Dying to this "Power of Sin":
By setting aside elementary principles, (Col. 2:8, decrees, laws and tradition), as Jesus did, (Mark 7:8-9, Matt. 15:7-9), Christians enjoin themselves with Christ, (Rom. 7:4), and die with him, (Col. 2:20), dead to the Power of Sin, (its laws, decrees, traditions), no longer have authority over Christians, (Rom. 6:11).
By raising Christ from the dead, it was proven that death no longer had mastery over Christ, (Rom. 6:9).
By trusting as Jesus did, in the work of God demonstrated at the Cross, in the love of God, his favor, forgiveness, and power, Christians are made alive, again, together with Christ, (Col. 2:13).
By being brought back to life, through Christ, death no longer has mastery over Christians, but life is given, even to mortal bodies, though his spirit who dwells within, (Rom 8:11).