Overflows are cavities and passages in the die which act as vents for air to escape and traps for excess metal flow.

Overflows reduce/prevent porosity in the die casting and promote complete metal fill into the far sections of the cavity.

In the heat sink block, the overflows on each cavity are on the opposite side of the gate. The overflows are tabular in shape and positioned at the parting line.  

The overflow tabs on the two bottom cavities are connected by a vent line  to permit pressure equalization and to reduce back pressure in the die cavities.