Life Chisel

Restricting a sacrifice outlet to your own upkeep tells you exactly when this was printed. The conversion rate is generous: trade any creature for life equal to its toughness, no mana cost on the activation, no once-per-turn clause. The catch is the timing window, which collapses the card's strategic surface to almost nothing. You cannot respond to a removal spell by chiseling the creature for value. You cannot sacrifice a blocker to survive a combat step. You cannot rebuy an enters-the-battlefield trigger at a moment of your choosing. The only legal use is the slowest possible one: untap, decide on upkeep that a creature is worth more as life than as a body, and convert. Sacrifice outlets evolved past this restriction quickly, and the lineage from here to Phyrexian Altar, Viscera Seer, and the modern free-sac toolbox is a story of designers loosening exactly the clause that defines this card. It survives as a curio of the era when artifacts that did powerful things were assumed to need timing restrictions severe enough to make them nearly unusable, and when "during your upkeep" was a balancing lever Wizards reached for the way later designers would reach for tap symbols and summoning sickness.