Glossary
One-way smoke
In one line: A smoke placed so you can see the enemy's feet or body while they cannot see you.
What it means
A one-way is a precisely placed smoke, often on an edge or a height difference, so that one side can see through (or under, or around) it and the other cannot. Whoever uses it shoots enemies who cannot see them.
It needs exact positions and changes with maps and updates. Against a one-way, you avoid walking where you are visible, take another route or use utility that forces the enemy to move.
An example
The controller places a high smoke on the exit of a corridor: from inside the site you can see the feet of anyone coming, but they see nothing. The defenders shoot the feet showing under the smoke.