Does the act of sinning involve some kind of power or does it mean an absence, lack, or loss of power, or instead an act of free will or temptation with or without power?
Basically, it refers to our free will. God blessed us each with freedom, and He respects and preserves that gift of freedom which He gave us. He protects it even at the tremendous cost of His Son’s life. As He protects the life He has given us, He also protects the freedom which He has also given us. Freedom even to reject Him and the One He sent. Yet, He will never force someone to love him, as He desires very strongly that our love is a conscious and completely voluntary love. A compelled love is neither free nor pure, and there is nothing impure associated with God. Thus, He grants even those whom He knows will remain true to Him the freedom to reject Him.
We’re certainly enabled to sin by virtue of free will. We have the freedom to do so, IOW, while not having the right to do so, according to our faith. But the very freedom to sin, once exercised, actually results in slavery; sin has power over us, the power of sin rather than the power to sin. Motivation for sinning varies but is generally always related to the desire for some perceived good or another and is simply located in the will of man in any case. Sin is always a disorder, issuing from the disordered state of the sinner.
Humans, like any other entity, have final causes as part of our essence. Acting in such a way that fulfills those final causes is good in that it directs us toward our Creator. Unlike the majority of the universe, which has final causality but is not conscious, we are free (at least sometimes) to act in opposition to our final causes; that is what constitutes sin. In that sense it is an “absence” or “lack,” since it’s a failure to adhere to what is proper to our being.