I believe you, I just can't really wrap my head around the difference. In both cases the person would be giving up the contents of their mind to allow the police to access the evidence. To go sci-fi for a moment. It would be like if they could read a persons mind being found as not testifying against yourself. Fingerprint scanner unlocking I can see, forcing disclosure of knowledge not so much.
How about this:
Officer: We want to search your car. Give me your keys. I see them in your hand.
You: Fine *hands him the keys*
Officer: OK, where did you park it?
you: "Gee, I dont recall."
The first one is not testamentary. The second is.