Concussive shells is an upgrade for the “Marauder” unit in Starcraft 2, which adds slow (or snare as it’s known in AoE3 - RIP) to the unit’s attacks.
Would be cool if such an upgrade was added to a unique ranged unit sometime in the future, as it promotes micro in an implicit way, by allowing you to focus speedy low HP or important units you want to pick off, while less skilled players can continue to a-move.