Michael Cohen has testified that he’s cooperating with prosecutors still in the hopes they’ll ask a federal judge to reduce the sentence he’s already received.
Why? He's seeking a Rule 35 motion.
The Rule 35 motion is the post-sentencing equivalent of the “5K” letter of cooperation that was sought after by many of special counsel Robert Mueller's defendants who pleaded guilty before they were sentenced. Among the Mueller defendants, only Michael Flynn received a "5K" letter. Even Richard Pinedo, who helped investigators understand how the alleged Russian troll farm bought fake identities online and helped prosecutors identify the individuals they charged, didn’t get a "5K" letter from Mueller’s team.
Simply helping corroborate information to investigators isn’t typically enough to get a formal sentencing reduction letter. The cooperators have to provide “substantial assistance” to get a letter like this. “Substantial assistance” has a high legal threshold, which prosecutors have interpreted to mean the cooperator led them to another defendant or crime.