There are many ways to rig a topping lift. They normally don’t need the mechanical advantage a block could offer.
Rgranger’s diagram is for a topping lift that is adjustable at the forward end of the boom. So there would be two turning blocks to turn the TL at the aft end of the boom, and one turning block at the forward end of the boom to turn the TL down to be made fast in some manner.
The simplest is just a line (Or wire) attached at the mast head run to the end of the boom. The terminus there usually allows for adjustment of the length of the TL so that the boom height can be adjusted. Shortened when you want the boom higher as when you are not sailing. Longer when sailing so that only the sail is holding the boom up.
One my H356 the TL ran from the end of the boom up to the mast head where it went over a sheave and down to a cleat on the side of the mast. It was adjustable at the mast.
Rgranger’s diagram is for a topping lift that is adjustable at the forward end of the boom. So there would be two turning blocks to turn the TL at the aft end of the boom, and one turning block at the forward end of the boom to turn the TL down to be made fast in some manner.
The simplest is just a line (Or wire) attached at the mast head run to the end of the boom. The terminus there usually allows for adjustment of the length of the TL so that the boom height can be adjusted. Shortened when you want the boom higher as when you are not sailing. Longer when sailing so that only the sail is holding the boom up.
One my H356 the TL ran from the end of the boom up to the mast head where it went over a sheave and down to a cleat on the side of the mast. It was adjustable at the mast.