The reason why the plant behaves this way when it is topped is because the centre of growth control is located in the apical meristem or main shoot. The main shoot sends suppressive hormones down to the lower or axillary shoots which stops them from growing rapidly. This mechanism does not stop the lower branches from growing but as long as the main shoot is intact it will be largely favored as the plant increases in height.
By removing the main shoot, the branches beneath it become free to grow at full rate in order to take its place. Please note that when the top shot is removed, the plant will no longer continue to grow in that location, so you have to make sure that there are enough secondary shoots to make up for the loss. It communicates with photosensitive pigments located in the leaves. The information that these pigments receive dictates the behavior of the plant. Shoots on branches that are in the shade will be supplied with growth hormones in order to elongate and catch up with the rest and this might waste precious energy. The first part of the system is called the Circadian Rhythm, which is basically an internal biological clock. This biological clock is an evolutionary response to light and darkness and is tightly linked with hormonal functions in the plant. Certain things are supposed to trigger at certain times of the day but also at certain times of the year.
The second part involves hormone signaling mechanisms, messenger molecules and specifically encoded proteins that tell the plant to start budding based on the information that it receives from the environment. The plant knows when to trigger flowering because the sensory pigments in the leaves keep track of the photoperiod or the hours of daylight and relay this information to plant. The spectrum of the sunlight also changes with the seasons, which provides the plant with the information that it needs to trigger flowering. The sensory pigments also inform the plant of how much sunlight a certain part of the plant receives, which enables it to relocate energy and growth hormones to where they are needed. Shoots that are stuck in the shade will elongate and that means building more stem. This energy could have been spent on other things, like bud nodes, which is why we try to help the plant to become more productive by topping and training it. There are several types of hormones that regulate growth and behaviour. One of the most important growth hormones is called auxin. It originates in the main shoot and is part of a mechanism called the auxin transport system. This hormone plays a big part in the internal signaling and growth control mechanisms of the plant. It also regulates the formation and behavior of other growth hormones that are responsible for everything from root growth to the formation of flowers. By removing the main shoot, the communication between the leaves and the main shoot ends, effectively canceling out the apical dominance. The result is that the plant assigns the next shoots in line to the job. This means that the smaller shoots on the branches beneath the cut start growing faster and gain size. Since there is no more apical dominance, the plant will grow into a bush because the newly appointed main shoots all have equal priority. These shoots usually grow very slowly when the plant is left untopped. The new main shoots will in turn suppress the shoots that are located further down on the main stem, so sometimes it is best to top the plant several times in order for it to fan out properly. Some say that it is best to top the plant at night when most of the hormones have been sent to the roots. This means that there is a smaller chance of the plant being stunted after the main shoot has been removed. There will be a short period of time when the plant is in something that could be called a state of confusion. It will stop all activity until it can figure out what is going on. It will resume vegetative growth as soon as the hormonal functions are up and running again and the dominant shoots have been appointed. It should take no more than a few days for this to happen, a week at the most. Most of the time this transition is quite fast but some plants that respond poorly to topping might display stunted growth for a while. It is possible to top a plant many times, each time the number of dominant shoots will double.
Give your plants some time to grow before you top them. If they are topped too early they might get stunted for a while, mainly due to the loss of photosynthetic tissue. I do top them quite early sometimes as you can probably tell from the pictures that I have included. Go by your feelings, once the plants look strong enough you can start topping and training them. Look for secondary growth at the lower nodes, that usually means that it is safe to top the plant.
When the plant has formed the fifth pair of leaves, it should be ok for you to remove the main shoot. This is a good way of training the plant if one wants to make the most out of the space available. Topping is also a good way of slowing down plants that tend to stretch a lot as each time the plant is topped it will redirect energy to a greater number of dominant shoots. The new shoots will never grow as large as the untopped main shoot will but they will most likely yield more in total.