Speed Twin Preload Adjustment

Hi All
New speed twin owner here having had a sprint for the last few years.
Has transformed the devon lanes!
Love the bike.
Just trying to adjust preload for a pillion.
I have the triumph tool, which is an allen key.
The part of the shock to insert is just round.... Tool just spins obviously.
Am i supposed to turn the entire bottom of the shock? That doesn't feel right.
Thanks in advance

 

Crikey, i will just have to be braver!
Thank you for the quick reply!