Everyone covered the features of the TD. If you're looking for water temps at certain depths, it works great. I bought one last season for use out of my kayak with my Salmon-a-rama winnings. I only used it once in August when I went out and checked where the break was. Ran some FF and spoons below the break, and then a J Plug above the break. Was a slow day, ended up going 2-2, but both kings were over 20 lbs (20 and 24 lbs) and both hit my J Plug that I ran with leadcore above the break.
As a sidenote, many times if you have your fish finder set right, you can see that break. I'm looking forward to using it more extensively this year and will hopefully increase my catch rate even more out of the kayak. I attached a few pics of the thermocline(s) out of my kayak during SAR. When I was finding the big fish, they were hanging out on the bottom right in the thermocline. Pulled a 27.95, 23.45, and 23.55 on the Monday afternoon of Salmon-A-Rama. The big ones were around for a few days before the current stopped and the fish moved on (or out).
As far as "at the ball speed", I use my 12 oz torpedo rod as my dipsey. Make note of your rod angle when you're hooking up and repeat that rod angle.