Wiktionary
key log
n. 1 The log which, if removed, would free up the whole logjam. 2 The key issue or problem, which if (re)solved, would make the current task easy to complete. The issue around which the whole problem revolves.
Usage examples of "key log".
Sometimes the logs jammed up in the river and an expert was brought in to find the one log, the key log, which would free the jam when removed.
His hands were starting to sweat with a fear that an inadvertent tug would break the vine or that he wouldn't be able to release the key log to provide the crucial diversion.