Rolled up = window is in closed position, fully raised to top. Throw off: short for "throw off track" ...in this context, to prevent the dog from smelling them. Straining the leash: this means going so fast that the leash is stretched. This is metaphorical--if there is not actual leash it means going really fast. "she gets a hit" - in this context, hit is smelling a person. But generally it means hitting the target, meeting the objective, etc.

Upwind and downwind are words used to indicate the direction of the wind in relationship to where someone or something is situated. If the wind is blowing from North to South, upwind is the direction from which the wind is blowing. So upwind would be North and downwind would be South in this example. A dog trying to pick up a scent would do best if the scent was coming from upwind of that dog because the wind is blowing towards the dog. If you were trying to avoid the dog picking up your scent, then you would be best positioned downwind of that dog because the wind is first blowing past the dog and towards your direction. If you were upwind of the dog then your scent would blow in the direction of that dog.