You don't raid or read up on raid builds do you?

You are missing something. TTW can be triggered with the following debuffs (which can indeed be placed on a raid boss).

Warriors: Thunder Clap
Paladins: Judgements of the Just
Druids: Infected Wounds
Death Knight: Icy Touch

These abilities (to my knowledge) are part of the class's tanking rotations, so as long as you have a traditional tank, you will have TTW active. If you didn't take TTW, your missing out on a 12% damage boost. And pallies/druids.. yes the talents are optional, but from what I saw in the wowhead comments they are pretty standard tanking talents.

Ah, that would do it; looks like I didn't properly understand what constituted a "snare". Thanks for the clarification.

The only (?) snare in the game which worked on bosses in BC (almost all of them) was Slow. This led to the idea of stacking mages, one "Slow b***h" who kept it up full time at a personal DPS loss + others to benefit from it.

Blizzard quickly changed how it works. Now the snare counts as a snare debuff even if the target is immune to the effect of the snare, or even from a glyphed frostbolt where the snare is removed but still counts. Confusing? Yup.

The list Jurandr made does mean most tanks will keep up the snare to trigger Torment now, but other things will as well.

Deathknight - Icy Touch
Druid - Infected Wounds
Hunter - Concussive Shot, Concussive Barrage, Frost Trap, Wing Clip
Mage - Blast Wave, Cone of Cold, Frost Bolt, Improved Blizzard, Slow,
Priest - Mind Flay
Paladin - Avengers Shield, Judgement of the Just
Rogue - Blade Twisting, Crippling Poison, Deadly Throw,
Shaman - Earthbind Totem, Frost Shock
Warlock - Aftermath, Curse of Exhaustion
Warrior - Hamstring, Piercing Howl, Shield Bash, Thunder Clap

That's an old list and may not be updated. Some hunter pet abilities probably also count but I've never tested them.

Roots (e.g. nova) are a different debuff and do not trigger Torment. Mobs are immune or not immune to roots. There's no weird in-between.

Raid arcane, frost, and fireball (FFB doesn't have the points) builds will all take Torment currently.