So, Advanced Micro Devices (AMD) is having a moment. The chipmaker's stock is moving higher after hosting one of those classic Wall Street events—an analyst day—where companies try to convince the smart money that their future is even brighter than everyone thinks.

Here's the interesting part: AMD is trading just below what chart-watchers call a resistance level. Think of it as a price ceiling the stock has bumped its head on before. And in the short term, that ceiling might be the thing that decides whether this rally has legs or if it's about to take a breather.

There's an old trading saying you might have heard: "sell at former tops." It sounds like folk wisdom, but it's actually built on some pretty straightforward market psychology. Let's break it down.

Imagine you bought a stock at its all-time high. Then it drops. You're sitting on a loss, feeling a bit of regret, but you decide to hold on. You tell yourself, "If it just gets back to what I paid, I'm out." So when the price does rally back to that former peak, you—and everyone else who bought there—might be lining up to sell and break even.

If enough people have that same idea, all those sell orders collectively create a wall of resistance at that exact price level. The stock runs into a crowd of sellers all trying to get out at the same door.

Sometimes, the reaction at resistance gets a little messy. Anxious sellers start worrying that other sellers will offer shares at a slightly lower price to get their trades done first. Buyers, after all, will always take the cheapest offer. To avoid being left behind, these nervous sellers begin to undercut each other's prices. One person drops their ask by a few cents, another sees it and goes a cent lower, and suddenly it can turn into a race to the bottom—a snowball effect that pushes the price down.

That's the full picture behind "sell at former peaks." It's not just about remorseful sellers getting a second chance to exit. It's also about the fear among sellers that they might miss their chance if they don't act fast, which can accelerate a decline.

Right now, AMD is flirting with that kind of level. It's not clear whether the stock will reverse course—maybe the analyst day optimism plows right through the resistance. But some traders are definitely watching that ceiling, wondering if the old adage is about to play out once again.