Linear slot diffusers are usually installed along a window wall or a perimeter, and they were positioned for the room as drawn rather than the room as furnished. When a bed, a sofa or a desk ends up underneath one, the result is a cold line down the middle of the space.
The 1-way linear cover sends that discharge to one side of the slot instead of letting it fall where it falls. Pick the side with clear space and the air carries on doing its job somewhere less annoying.
The cover is 48 inches long, 6 inches wide and 2.75 inches deep, in polystyrene, holding magnetically on the diffuser or a 1-inch grid system. The manufacturer states it suits diffuser widths of roughly 2.75 to 4.75 inches.

What does a 1-way linear deflector do?

It closes one side of the slot discharge and opens the other. Instead of air spilling down or out both ways along the run, the whole output leaves on a single side. On a perimeter slot that usually means pushing the air toward the window and away from the room, or the reverse if the window side is where people sit.

Which side should it face?

Toward whichever side has room for the air to travel. Over a window and down the glass is the common choice on a perimeter run, since that is where the heat gain is and nobody is sitting there. Into the room is the right answer when the window side has a desk against it. What you want to avoid is sending the full discharge at a wall or a tall unit within a foot or two, because it turns and drops back down, and you have moved the draft rather than fixed it.

Will it fit your diffuser?

Measure the slot width first. The manufacturer states this cover suits diffuser widths of roughly 2.75 inches up to and including 4.75 inches. Outside that band it is the wrong part and no amount of goodwill will make it right. Then check the attachment surface, because the magnets hold on the diffuser or a 1-inch grid system and both have to be steel. Last, check you have 2.75 inches of clearance below the ceiling line.

What if your linear run is longer than 4 feet?

Most are, and this is the part people get wrong at the ordering stage. Each cover is 48 inches, so a 12-foot run takes three and a 10-foot run takes two with a 2-foot gap you need to decide about. Work out the count and where the joins land before ordering rather than discovering it on the ladder. A gap in the middle of a run is usually less noticeable than a gap at one end, but that depends on where people actually sit.

How does it install?

Offer it up to the slot with the open side facing the direction you chose, and let the magnets take it. No tools, no fixings, nothing to make good afterwards. A 4-foot part is awkward on your own, so a second pair of hands helps more than any tool would. Once it is up, stand in the spot that prompted the complaint and check the result before you move the ladder.

Does redirecting a linear diffuser cause problems elsewhere?

At one run, rarely. The slot passes the same air, it simply leaves on one side. The thing to watch is a long perimeter where you cover several sections and push all of it the same way, because the combined stream can carry further than you expect along a flat ceiling. If you are treating a whole elevation rather than one problem spot, it is worth a word with whoever maintains the system.

Is this a filter?

No. It changes direction, not air quality. There is no filter media, no MERV rating and no filtration claim. If you want to clean the air rather than aim it, that is a different product.