The lack of pressure of it being in the collar during a striking. It is sometimes referred to as the third die. The metal would flow toward all three surfaces, obverse, reverse and the edge when struck.
Keep in mind that under tons of pressure, the metal flows. As a die ages and the surfaces and details/devices wear, the flow begins to take on patterns and flow irregularly. Striated surfaces and fuzzy details. But when the dies are new and sharp, where surfaces are smooth and details sharp, the ‘mold’ gets filled out better and hide most irregularities with a planchet.
