Holding-time reversal along a fixed CTMC jump path
= Holding-time reversal along a fixed CTMC jump path
Conditioned on a finite jump path $i_0,\ldots,i_n$, the holding times are independent exponentials with the corresponding rates. Multiplying the probability of occupying $i_n$ at time $t$ by $q_{i_n}$ turns its simplex density into a product symmetric under reversal of the time portions and the state sequence.