This Express Lane System Will Get you In the Swim FAST The 2 mph system is for the light or casual swimmer. It will create enough of a flow so that you can gently swim against it without exerting your self to a great degree. It's also great for water therapy and resistance walking. The 4mph system offers a stronger stream for a great invigorating workout. This system is perfect for the moderate swimmer, giving a fast steady pace for that work out you have been looking for. The 6mph Dual system will give you all you can handle. With a very strong stream for the advanced swimmer looking to increase their endurance or add to their muscle tone. NOT for the beginning or moderate swimmer. Trim - Spa Technologies