The original idea of “trust index” was that each person in Ripple could set how much they would “recycle”, a percentage, and that behaviour would somehow propagate, spreading along with IOUs. Your “trust index” would imprint itself on the credit lines you made, and propagate “swarm redistribution” relative to the trust index that was used.
With symmetric swarm redistribution, that becomes quite trivial. Since each person is propagating a “pulse” of forgiving debt, and forgiving a total amount of debt equal to what they were forgiven, the “trust index” can be used so that if two people have debt to you, and one has a credit line with an index of 2%, the other of 1%, then the first one will receive 2x as much as the other one, via symmetric swarm redistribution. They will then continue the “pulse” based on the total amount of debt they were forgiven, and based on the trust indices of people who have credit lines to them.