Bargain picatinny to arca adaptor

I Picked up one of those, and one of these and this. The combination is way steadier than I am. I really didn't need the Picatinny to ARCA but, I think I can use it on another rifle.

View attachment 358912View attachment 358913
How do you use the two tighten knobs, use both or just one. Does it lock in place? I have had ball heads for years, Manfotto, they never lock in place the weight of cameras always shift a little.