In the early stages of teaching cello to a beginner, any good teacher is concerned with shaping the student's hands in a rational manner, i.e. fingers curved and not buckled, joints flexible and not locked.
Much has been written about playing the cello "naturally." My feeling is that the most "natural" thing we could do is to shape our cello-playing and bow-holding hands according to what our hands themselves want to do, rather than according to a preconceived idea of what hand positions ought to look like.
The most relaxed and natural state for our hands and arms to be in is hanging loosely by our sides. Here are four different views of my left hand doing just this.
Things to notice:
The fingers want to be curved, not straightened
The fingers are naturally spaced. The amount of spacing varies a lot between individuals.
The fingers want to hang down in a slanted way.
The thumb opposes all the fingers, rather than any one particular finger
The thumb opposes the fingers at a "sideways" angle
Now let's bend our elbows and bring this "natural" hand up to face us...
...and you'll notice that it looks remarkably like a "violin hand"! Remember this, it's important. (Mats Lidstrom has written that his teacher, Leonard Rose, told him "You’ll learn as much from your violinist friends as you will from me.")
Turn your arm around to make a "cello hand"...
...and the last step is to replicate this with your cello.
Things to notice:
The fingers are slanted (pronated) on the string, per their natural slant when hanging down
The thumb touches the neck of the cello "sideways" rather than flat on the pad. This prevents a student pushing the pad of the thumb into the neck, buckling the top joint, creating tension and inhibiting ease of shifting.
Now let's do the same thing with the right hand.