You're given a letter board and a list of candidate words. Find every word from the list that can be traced through horizontally or vertically adjacent cells, using each cell at most once per word. Searching the board separately and completely for each word finds every match, but words sharing a common prefix pay for that shared prefix's board steps over and over — once per word. Inserting every word into a trie first, then walking the board exactly once while moving through the trie in lockstep, means a shared prefix is only ever traversed a single time: the DFS reaching a trie node marked as a complete word records it and simply continues from that same node, since it might also be partway through a longer word.