Thans for the tip! I need more practice. ;-)In theory the method is very simple but in reality it's harder to do than it seems.
You need to have a displacement forward the subject and you need to stay linear. No big matter is you shift a bit but always stay linear. You must take care that regarding your linear move, the subject not rotate or move.
To do this a shoot in bursts with flash at 1/16th of power.
There is a lot of photos thrown to trash using this method because our subject is alive and moves sometimes.
The best time to do focus stacking handheld os when insects are sleeping. Before the sunrise (need to wake up early) or after the sunset.