Recently, an interesting phenomenon has emerged in the market structure — it's not just about large gains, but three issues stacking up simultaneously.

First, valuations are at high levels; second, a few mega-cap stocks have excessive weight; third, these heavy holdings have shown particularly fierce upward momentum recently. This combination sounds a bit familiar because historically, whenever the market is "overextended," these characteristics tend to appear. Even more painfully, before large-scale corrections, such signs are usually visible.

Where is the problem? When the index increasingly relies on a few giants to drive the rally, if these companies' profits fail to meet expectations, interest rates rise, or policy fluctuations occur, the decline can come very quickly, very concentrated, and very hard to avoid. Those seemingly steady index gains may not withstand any unexpected shocks.