In the settings it can be selected which indicator is used, is selected by default.
Length of the arrows is determined by the strength of the momentum:
Stochastic: Difference between D and K
RSI: Difference from RSI-50
WaveTrend: Difference between the Waves
(Thanks to @LazyBear for the WaveTrend inspiration)
If anyone has an idea how to conditionally change the color of the arrows, then please let me know - that would be the icing on the cake. Then it would be possible to indicate Overbought/Oversold levels with different colors.
Unfortunately it currently seems not to be possible to dynamically change the arrow colour.