On the primary DFS projection page, R2 presents estimates of the probability that a player will appear in an optimal lineup based on the results of 10,000 simulations.  In those simulations, the projections of quarterbacks and their receivers are uncorrelated.  However, in DFS you want to take advantage of correlations between players to give your roster a higher ceiling.  A primary method to achieve this is to stack a quarterback in the same lineup with one or more of his receivers.  To estimate the probability that a QB and WR/TE tandem appear in the optimal lineup that week, we simulate the week again with some correlation between QB performance and WR performance using a coefficient we estimated empirically.