Shabbat 130 - Preparing for a circumcision
Circumcision should be done on the eighth day of a baby's life, and it can be done even on Shabbat - because the Torah required it to be done in its proper time. If one forgot to bring a circumcision knife before Shabbat, then normally, since carrying anything in a public area is prohibited, he cannot bring this knife now. Bringing the knife is not a mitzvah per se, but only a preparation for it.
However, Rabbi Eliezer argues against exactly this point. In his opinion, preparation for a mitzvah is just as important is the mitzvah itself. In fact, in the place of Rabbi Eliezer they would go so far as to fell trees to prepare charcoal to make a knife on Shabbat, for the purpose of the circumcision. And, even though for a Shabbat violation one gets cut off from his spiritual source and may have his life reduced, in the place of Rabbi Eliezer everybody would live out a long life.
When one, following Rabbi Eliezer, brings a knife on Shabbat, he should be showing it to everybody, to emphasize the love for the mitzvah. In the time of danger, when circumcision is prohibited, he can cover it, but do it in the presence of witnesses, so that the love for the mitzvah is still known.
Art: Alexandre Gabriel Decamps - The Knife grinder