[lrm242]

Sell side traders or block crossing networks (where the other side is taken by another block trader).

Sell siders are compensated because they will build into the price the expected impact. The pension fund gets a known price impact with essentially 0 variance and the sell sider takes on the risk of execution to offset the trade, and essentially earn a spread between what the pension fund paid them for the risk transfer and their 'skill' in getting the trade done in the market.

It obviously can and will be more complicated than that with varying benchmarks and compensation schemes, but that is the jist of it.