When your opponent goes to his knees in judo, the ref will usually call “matte” and give him a stalling penalty. Otherwise, it’s considered that they entered newaza (ground work). There’s a little hazy area (in terms of the refs having to make a judgement call) here if the opponent is on his knees but starts standing back up. The two videos below show cases when the refs give an ippon for an attack in such a situation. I can see an ippon for Tony in the second video, but I honestly can’t see an ippon for Anai in the first video. Either way, it seems wise, in general, to stay turtled up and take the penalty if your opponent still has a grip on you.