It’s a combination of things really; forearm pressure on the bass body from the picking/plucking hand is one. Sometimes I like to warm up by fretting without touching the back of the neck at all. If the bass is set up well, this should be possible.
I also commonly will have my thumb pointing more towards the headstock and more of my fretting hand comes in to play then, but it’s not something I would describe as great technique to emulate.
It should take only a little pressure to fret a note.
@howard I literally discovered this at the tail end of Module 2 and in Module 3, and especially the first time finger crossing learning Sweet Home Alabama. I was so concentrated on getting the plucking right, I look over and my thumb wasn’t even touching the neck!!!
Thanks for the tip, I just did that sitting here writing this reply and boom! My fretting fingers straightened out.