This exercise could be named "red light/green light". Essentially what you are teaching the dog is that a taut leash is a 'red light' and causes the walk to come to a grinding halt, while a loose leash is a 'green light' and allows the dog to move forward. There are two parts to this exercise. The first part alone may work for some dogs, while others may need both parts to get the message through.
Walk with your dog and the second the leash goes taut, stop in your tracks. Now comes the waiting game. Wait until the dog moves in such a way as to create some slack in the leash, then immediately move forward. With enough repetitions, the dog learns that every time the leash goes taut, the fun walk stops, and he also learns that he is the one who has control over that.
For my Vizsla, I found that it didn't bother her to stop and stand there for what seemed like hours on end. She'd happily keep looking ahead and keep the tension on the leash. So with her, I had to go to the next step. Once you've stopped, if the dog doesn't make the leash go slack within a reasonable time frame, turn and walk away back the way you came from, taking the dog with you. Again, so that you can pull him with you, walk away at an angle at first until the dog is turned around. Once the dog is coming with you and the leash is loose, and before he lunges on ahead in the new direction, turn and resume the walk in the direction you were originally going. You can praise or treat here if you like, but with many dogs, just being able to continue the walk is reward enough for them. As Jean Donaldson writes, "A good motto is 'We'll keep doing this patch of sidewalk until you do it without pulling.'"