Maybe tachyon weapons work in a way similar to what tachyons are. Tachyons are supposed to particles that travel faster than light, and therefore have an inherited accelerated time travel abilities, something like built in 4dfx card. Maybe by decelerating them you alter their 4dfx abilities in a away that they cause damage. It could be death by gravity.
Another answer could be this: Since tachyons naturally move faster than light, they will try to recapture their speed after deceleration. Since this takes lots of energy, the particles take it from wherever they can get it, i.e. the atoms that make up the hull of a ship.
In each case, the tachyon guns cause "sub-nuclear" or quantum damage.
[Edited by Meson on 06-12-2001 at 03:54]