Sanhedrin 17a (translation modified from Sefaria):
אמר רב כהנא סנהדרי שראו כולן לחובה פוטרין אותו מ"ט כיון דגמירי הלנת דין למעבד ליה זכותא והני תו לא חזו ליה
Rav Kahana says: In a Sanhedrin where all the judges saw fit to convict the defendant, they acquit him. What is the reasoning? Since it is learned as a tradition that suspension of the trial overnight is necessary in order to create a possibility of acquittal, and as those judges all saw fit to convict him they will not see any further possibility to acquit him.
The case here is the initial discussions about whether he is guilty or innocent. Everyone is supposed to try to find a way to acquit the person tried, and they wouldn't decide the outcome until the next day so that they could consider the arguments for acquittal (see Sanhedrin 4:1). If no one had any argument to acquit, then he would be acquitted automatically, because they weren't able to consider overnight any arguments in favor of his innocence. If the 23rd judge thinks the defendant is guilty, he can't just vote to acquit him, because the whole point of the discussion was to bring up arguments for acquittal; he has to actually have an actual argument for acquittal so that the other judges can consider it overnight. And if you do find an argument for acquittal, that's the whole point of the process.