Avocado plants from seeds
Gardening Reference » Gardening in 2002
Avocado plants that are grown indoors tend to grow rather tall and spindly. They do not branch unless they are pinched or cut back.
To pinch back, simple pinch out the newest pair of leaves as they emerge. Usually this is sufficient to cause the plant to send out two branches at the pinch-point. After those become longer, they can also be pinched so that they branch out.
BTW, you cannot pinch a 12 inch stem back to 6 inches. That is pruning, not pinching.
To help your avocado develop sturdier stems, provide as much light as possible.