Shock content deploys extreme, graphic, disgusting, or taboo-violating material to seize attention through a strong involuntary reaction — you look and react before you choose to. Gore, threat, disgust, and taboo violation trigger fast, automatic orienting and arousal, and that arousal is then attached to a brand, a click, an ideology, or an act of sharing.
The observable move is presenting material whose extremity exceeds what the message actually requires: graphic injury in an unrelated ad, a lurid thumbnail on a mundane video, an atrocity image bolted to a political claim. Because disgust and horror are sticky and memorable, the content spreads and lingers even when the viewer disapproves. It differs from a neutral or pleasant surprise, and from ordinary emotional imagery, by pushing past comfort thresholds.
The manipulation is the gratuitousness — extremity beyond informational need, used to buy attention. The strong reaction is the lure, and sharing while activated is what the content is built to extract.