Down Pass (Computing subtree answers)

Pick node $1$ as the arbitrary root. Run DFS. For each node $v$, compute down[v] = the answer for the subtree rooted at $v$. This uses standard tree DP. This is tree DP. You combine answers from all children of $v$ to get down[v]. The recurrence depends on the problem. For sum of distances, you add child contributions plus edge costs. After this pass, you know down[1] = the answer when node $1$ is root.

But you still need answers for all other roots. That is what the second pass does.