Hi, this is my first topic on the forum.
I've been having some trouble with my left hand. I come from playing guitar and they always taught me to have a relaxed hand, the more relaxed the fingers and close to the fingerboard, the better. But now that I'm playing the cello my teacher tells me I have a "weak" fingering, that I should tap on the fingerboard and hear the knock of the tip.
https://www.youtube.com/watch?v=QoHOzEDvCj0 for example, in this video at 1:10 you can clearly hear the left fingers tapping onto the fingerboard.

I was wondering what kind of exercises should I do to strengthen or if it's not a matter of strength but the conception of how my left hand works please help

Thanks!