In CS:GO, smokes were static gray domes. In CS2, smokes are volumetric entities. This means they expand to fill the geometry of the map. If you throw a smoke into a small tunnel, it will push outward and fill the entire space, leaving no gaps.
The HE Grenade Clear Because smokes are now dynamic, they react to explosions. Throwing a High Explosive (HE) grenade directly into a smoke will temporarily clear the gas, giving you a 2-second window of perfect visibility.
The Aggressive Clear: Throw an HE into a chokepoint smoke and immediately peek through the clearing to catch enemies holding W.
The Info Clear: Use an HE on a default smoke (like Mirage Window) simply to gather information on enemy positioning without committing your body.
Note on Bullet Tracers: Shooting through a smoke will also carve small, temporary holes in the gas. If you spam a smoke, enemies can see exactly where you are standing.