They will branch from the top of the stump after they are beheaded. This behavior is presumably faster and more vigorous the closer the cut is to the tip. So cutting near the base of an old plant may be dicey.
I have beheaded plants on purpose in order to force branching (removing the top inch or so) and I was pleased with the outcome. The response is very similar to what happens when they flower, when the growth point has turned into an inflorescence. A headless plant will likely not be headless for long, given permissive conditions and reasonable vigor beforehand.
Starting Pachypodium cuttings is not a whole lot different from starting cactus or other succulent cuttings. You have to give them time to heal before potting them up. Some cuttings will leak sap for a while and this may create a big black stain, so put something underneath while they heal.
I like to go in with nail scissors and remove any spines around the bottom of the cutting at the base. Just the spine. This is because as the base of the cutting heals, it sort of shrinks inward, and as a result a spine at the bottom of the stem would start to point downward and get in the way down the road.
Pot the cutting up in a relatively small pot (not a deep one) in fast draining mix (like 50% pumice or gritty equivalent). The base of the stem should be in contact with the soil but very little stem should be below soil level, the bare minimum needed to keep the cutting upright. Maybe put a rock on either side or stake the cutting to help with stability while it is rootless. But don't bury the stem too far.
Water with restraint at the beginning, more or less when the soil has gone dry or almost dry. At some point you will begin to note activity on top, and that is a sign that you can increase watering. These plants are thirsty during the warmer months when they have roots... I water rooted, established Pachypodiums twice as often as the average succulent while they are in leaf.
Indoor plants are going to require and consume much less water than a plant out in the sun and the wind, so don't go crazy with the water in the months after you pot up a cutting, if it will be indoors.
Segments may root and sprout new branches. I've never tried but I would imagine it's quite possible, if conditions are entirely favorable. Make sure you keep a upright orientation (not upside down).
Above all you will get the best results (forcing branching and rooting cuttings) if you take action at the start of the growth season, or close to it. This time of year would be the absolute worst possible, so think and plan and prepare, then execute in six months or whatever.
