Words have power.
Jewish people are constantly giving blessings. A boy or girl is born. “Mazal Tov!” / A good fortune! An engagement! “You should have lots of Nachas / Nachat!” (Satisfaction) or “Build a Bayit Ne’eman Be Yisrael” / faithful home among the people of Israel!
When they meet. “Hatzlacha! / Success!” You hear someone got a new job. “Mazal Tov! Parnassa Tova!” / A good income! A Bar Mitzvah! “He should grow up to be a big Talmid Chacham!” / Torah Scholar. At a Brit / Bris Milah / Circumcision! She Yizkeh leGodlo beTorah, Huppah UMaasim Tovim. You should merit bring him up with Torah, Marriage and good Deeds.
What is a common answer to a blessing? “Thank You!” or “You Too.”
What is this like – A person gives you a check for $1,000,000. You say thank you very much. You don’t deposit it. You don’t endorse it. You don’t use it. One day you wake up and find the check hidden at the end of your office supplies drawer. “Wow I never deposited it?” You look at the date – 6 Months have passed. You don’t remember the giver. You really need the money. I guess you’ll learn for next time.
A Blessing has the power to affect the positive or negative. Prayer to G-d / Hash-m will have some kind of positive effect – either on the person you pray for or another person.
But you can make it more effective by “endorsing the check.” A blessing is a bit like a check – if you endorse and deposit it, it will appear in your account.
How do you endorse a blessing? You say “Amen.”
To return the blessing – we usually reply – “Vekhen LeMar” וכן למר
This means “and also for the master” implying the person who made the blessing.
So you receive the Blessing. You say “Amen. VeKhen Lemar.”
Now since they are recieving the same blessing of “Vekhen LeMar” The person who initially gave the blessing should also respond “Amen.”