Calculate the delta

Good Afternoon
Firstly I want to say thank you for creating robust forecasting machine
Recently I studied about Prophet
I confuse how Prophet calculate the delta (change magnitude) in the past data
I have try to read the documentation and source code but I still can't understand

The documentation of Prophet(page Trend Changepoint) says
"It then puts a sparse prior on the magnitudes of the rate changes (equivalent to L1 regularization)"
Can you show me where do this process happen in the source code?
I really need your help