What a sniff walk is
On a sniff walk, you go slowly and let your dog investigate. You might cover a few hundred feet in twenty minutes. That is fine. The goal is not exercise. The goal is for your dog to use the sense they rely on most.
Think about what a dog learns from one patch of grass. Who passed by, how recently, and in many cases, how they were feeling. We cannot know all of it. But we do know that sniffing is a big part of how dogs gather information, and that a walk with no chance to sniff leaves a lot unfinished.
Many dogs also seem calmer after. The research on this is still developing, so we will not promise a result. In our experience, a dog who has been allowed to sniff often comes home more settled than one who has been marched around the block. For more on that, see how sniffing calms dogs.
A good test is simple. At the end of the walk, ask whether your dog got to finish the things they started. If the answer is mostly yes, you had a sniff walk. If you were pulling them away from every interesting spot, you had an errand.
You can do both in one outing. Spend the first ten minutes getting to a good patch of grass, then clip on the long line and let the walk begin. Plan for the quiet part to be the main event.