Dave's description matches mine exactly. Though (and I am not at the boat) the loop to which you tie the reefing line may be in the track on the bottom side of the boom rather than riveted to its side.
Using a small swivel block at the bottom end of the topping lift (set so it is about 2 or 3 feet above the boom when the boom is level) helps 1) raise and lower the boom and 2) lets you remove the boom from the topping lift more easily. The line that goes through the block routes into the boom at the front end through one of the 3 sheaves (through the jam cleat up there), then through the boom to the aft end, through the matching turning sheave at the aft end, up to and through that swivel block, then down to be tied to the "bar" above the turning sheave. The mechanical advantage from that block is useful. End of season, you just untie the line from the "bar" and pull out from the block, leaving the topping lift itself attached to the mast crane.
Using a small swivel block at the bottom end of the topping lift (set so it is about 2 or 3 feet above the boom when the boom is level) helps 1) raise and lower the boom and 2) lets you remove the boom from the topping lift more easily. The line that goes through the block routes into the boom at the front end through one of the 3 sheaves (through the jam cleat up there), then through the boom to the aft end, through the matching turning sheave at the aft end, up to and through that swivel block, then down to be tied to the "bar" above the turning sheave. The mechanical advantage from that block is useful. End of season, you just untie the line from the "bar" and pull out from the block, leaving the topping lift itself attached to the mast crane.