What is a tactical exercise without troops?

It is a wargame played by soldiers as training. They will plan a battle from maps, aerial reconnaissance photographs, etc., then often tour the ground the exercise is drawn from and revise their plans. They may play out the scenario with some sort of rules and one or more umpires. A TEWT can be run with many or a few players. Yes, the acronym is TEWT.

Have you done one?

Of a fashion. I am not and never have been a member of any armed force. On a battlefield tour in Normandy in university, we did a planning exercise from maps, aerial reconnaissance photos, and a walk around Putot-en-Bessin (it was a little corrupted in that I had read the relevant section of the guidebook beforehand). I have also run a (halting and abbreviated) session of the British Army Tactical Wargame (1956 – http://www.wargaming.co/professional/details/britisharmy1956.htm).