Most people, when eating corn, will hold the corn parallel to their teeth. They will move the ear closer their face and take bites to remove kernels in sections without actually biting in to the cob, usually going left to right and then rotating the cob to restart the process OR rotating the cob and then moving to the left or right before restarting the rotating.
The sign is instead suggesting that Goldberg introduces the corn to his mouth in a perpendicular fashion, inserting the whole ear in past his lips. The implication is that it simulates oral sex on another male, which implies an insult to his masculinity in the culture that this was prevalent in. It is also a fascinatingly stupid way to eat corn.