This actually is a very simple question to answer. When you receive Communion, Jesus is present in you physically for approximately 15 minutes, but spiritually He remains with you till death. Once you’ve made your first Holy Communion, He leaves something of Himself in you that is in your heart. He is also residing in you from your Baptism, and your Penance and your Confirmation, if you have it. He dwells in you. He never leaves. He is in your Heart. This is the place the mystics go to seek Him to find union with Him during prayer. He is waiting there in your heart for you to seek Him. That is how some of the Saints find Heaven on earth through prayer experiences that go deep. Try it.
When you pray the Divine Mercy Chaplet, you are drawing from this reserve of the Eucharist in your soul and offering it to God through that particular prayer: Eternal Father I offer You the Body, Blood Soul and Divinity of Your dearly beloved Son our Lord, Jesus Christ,… It is from that place within that you are making the offering from. Kinda like and Act of Spiritual Communion in reverse. Get it? You can actually receive Communion on some one else’s behalf and ask at that time that they receive the graces you beg for them when you talk to Him at Communion time. It is very powerful to do so.
I hope this helps.
Glenda