Step 1: Remember the riser's whole job.
A riser exists purely to feed extra liquid metal into the casting as it shrinks during solidification, compensating for the volume lost when liquid turns to solid so that no voids form inside the part.
Step 2: Ask what the riser needs in order to keep feeding.
To keep supplying liquid metal right up until the very last moment, the riser itself must still be molten for as long as the casting is still solidifying, if the riser freezes even a little early, it can no longer push any more metal into the casting and the last liquid pockets inside the casting are left starved, forming shrinkage cavities.
Step 3: State the required solidification order.
This means the riser has to be the last part of the whole system to turn solid, in other words it solidifies only after the casting itself has fully solidified. Designers achieve this deliberately by giving the riser a larger volume to surface area ratio, following Chvorinov's rule, so that it naturally cools and freezes more slowly than the casting.
\[ \boxed{\text{After casting solidifies}} \]