Here's an old article I saved which explains it thoroughly:
https://mega.nz/file/RAwRTT6T#5RH8jomqT ... ayZk1znVRA
That tradition is still going on and those birches (pomlazka) are sold openly on market all over Czechia.
They are provided in a number of lenghts and are surprisingly flexible. Used on clothing, especially jeans, it will take some force to cause any pain at all (I can tell because I was so lucky to get my hands on some real and fresh pomlazka
So let's use this thread to collect some real-life examples:
A very old clip
https://streamable.com/w2mtxd
Pics from my collection