No definable memories, only marginal pain...it was progress, though Severus was not about to dispense with any compliments. He could feel the semi-demonic presence, which was likely Winchester's dormant ability and opened his own magical floodgate in return. Once it was clear to him that Winchester intended to fight back, it was only logical that he give the other man something to truly fight.
A presence moving for his mental fortifications was not enough and Severus knew that - any presence could do just that, but Severus was looking to initiate a push. Some form of motivation to fight back with little restraint, so he twisted the magic until it resembled a snake. He could of course have chosen something seen within Winchester's mind, but he didn't need it. He'd already seen it, within Meg, each and every time he'd crossed paths with her and planned to use it to evoke some form of reaction.
When the snake opened its eyes, they were yellow and sought to burn through whatever meager defence Winchester had.