TF +dmg gives damage to base druid attack damage. so basically its listed char sheet damage plus whatever TF is worth. in cat form, this is basically a flat DPS increase, since the normal attack in cat is 1.0 spd.
this new base damage (call it TFdmg) is then used as the base damage from which shred, claw, mangle, and ravage are derived from. hence, with shred, youre getting that 140% (or whatever it is) modifier. with ravage, you get that 280% (or again, whatever it is....i cant remember off the top of my head). with the mangle debuff granting +30% damage to shred, adding TF into your shred rotation increases the damage you get from it by a much larger amount that most think. if TF adds 40 damage, shreds 140% modifier turns that into 106 dmg more or so, which then gets modified further by 30% from mangle, for about 35 extra damage.
so ultimately, with shred and mangle modifiers, the 30 energy cost of TF (which was 1.3:1 DPE) now becomes a boost of 181 dmg instead of 40 (or a DPE of ~6:1).
thats not factoring in crits, or the +crit dmg boost talent, which further enhance the relative DPE contribution of TF.
essentially, with TF, you need to factor in the modifiers of the skills. on normal attacks, TF max rank corresponds to an AP increase of about 560 AP (its +40 dmg/sec, or 40 dps, and 14 AP = 1 dps, so 40*14). this "extra" AP is then factored, as base damage, into the attacks of your cat form druid. anything with a modifier on it (mangle, shred, ravage) receives a much bigger increase from TF than a mere 40 damage. also keep in mind that 560 AP is a bigger increase than the max rank of rampage, the warrior 41-pt fury talent, or even the highest rank of talented battle shout.