Most people have little idea of the magic that keeps them upright. Unfortunately that is of little help when teaching people to ride. The answer is fairly straightforward. It is impossible to ride a bike in a perfect straight line with the centre of gravity over the line. You will fall off at any speed (including stationary). Instead as you move forward you subconciously steer into the fall. This causes the centre of gravity to pass over the line and you will correct in the opposite direction. The reason it gets easier with speed is that the centripetal force increases with speed, so less correction is needed.
Sadly, gyroscopic effects have almost zero contribution, as wsa demonstrated using a bike with counter-rotating extra wheels.
Now I have broken the magician’s code, at least you can explain to the three year old how to balance their bike! It also helps to hold their handlebars as you walk alongside, turning into the direction of fall.
PS is you don’t believe me, feel free to brace your handlebars and ride off, just don’t blame me for the consequences.