That is OUR part We must rend and OPEN up our hearts to Him. And God's part is mentioned in the very same chapter in Joel. It breaks the shell of the barrier that separates our lives from His Spirit. Our hearts and souls then bleed forth, so to speak, in sorrow for our sins. These sins separated us from God as flesh upon the hands of two parties separates the blood from uniting in the two.
Sin is in our flesh.
I that do it, but sin that dwelleth in me . I find not. We do not literally cut our flesh, but this occurs spiritually when we confess our sins and sorrow over the sins we have done. It is as though our hands are rent and we clasp Jesus, and mingle our lives with His. We are totally genuine with God and ourselves at this point.