According to details posted on the Heavy update site, the Sandvich—which replaces the Heavy's shotgun—is unlimited in quantity and heals 120 health for the Heavy per use. During the four seconds required for the Heavy to eat the Sandvich, the brute is completely vulnerable, drawing enemies with the loud sounds of vigorous chewing.
The unlockable was hinted at in the Heavy's class-specific achievements, which were unveiled last week. Among them is the achievement "Konspicuous Konsumption," which requires players to eat 100 Sandviches.
The Sandvich marks the final piece of content in Valve's expansive Heavy update, joining the other two new unlockables Nastacha and the K.G.B. The Heavy update will also bring Arena mode, the new Payload map Badwater Basin and a new community-created map when it becomes available later this afternoon.