Tactics aren't calculations?
Well yeah you always have to calculate but I think when you see the pieces moving you don't look as far as you would if you didn't move them. So you get the same kind of training you might get with shorter tactics puzzles.
But if you don't move the pieces you end up calculating farther ahead and I think this is better for improving at that.
There's some nice stuff here:
http://youtu.be/vvPcVwrZ4nI (1:29:59) Melborne Chess Club
http://youtu.be/tySL8qEVwhU (40:28) St. Louis Chess Club
Tactics aren't calculations?