Most contracts are predicated on the supposition that the defenders will win some tricks. If game can be made with "26 points and eight trumps," this leaves 14 points and five trumps to the defenders as an "allowance."
The defenders' 14 high card points gives them an "allowance" of at least three tricks. (They actually have the material for four tricks, but it is assumed that the declarer will be able to "ruff," or otherwise neutralize one of them.)
Suppose one defender has JT98x of trumps. He has the whole trump "allowance" for his side. More to the point, this holding will take two trump tricks, whereas if they were split 3-2 (which occurs 68% of the time), these five trumps would take zero tricks.
Absent the jack of trumps, the defenders have an "allowance" of 12-13 high card points which should be good for three tricks. Their two trump tricks are clearly above their "allowance." Does this fact make this a good time to double for penalties?
And how do you guard against the possibility that the double will scare the opponents into a better contract, or that revealing where your strength lies would cause declarer to alter his play and make the contract?