There are multiple possible ways how this passage (Jupiter, bars 170-190 that would be, I presume) could be played. I'll discuss three of them:
Two-hands, single-finger, single-string
Here's a rendition of the passage where you can clearly see them alternating with both hands, each using just a single finger. So that would perhaps be the “correct answer”.
I actually find this really difficult coördination-wise, if actually thinking about plucking the strings. It does work really well though when I'm more thinking about merely tapping on a drum head, combined with only a slight twisting motion to pluck the string.
Well, I wouldn't have though of this myself. I would have tried other techniques (which, if nothing else, have the advantage of being applicable to something that's not merely open strings).
Single-hand, single-finger, single-string
The most sensible choice for this kind of stuff in general would IMO be to just lightly stroke back and forth over the string with a single finger. I find the thumb works best on cello (violinists use, I think, the middle finger – that's probably what the violists, who also play these sixteenths, would be most happy to do).
Again it helps if you don't even really think of plucking the strings – more act as if you were playing a light, slow arco tremolo, except you don't use the bow hairs but your finger.
This technique works quite well without putting the bow out of hand, you can just keep hold of it with the fingers while the thumb plays. Or, as I like to do for short-but-involved pizzicato stuff, rest the bow on my elbow. Actually I've never seen classical cellists do that – or, indeed, anybody but Nat Smith –
but it can really help a lot to keep the balance.
Single-hand, triple-finger, twin-strings
A really exotic alternative would be a multi-finger pattern over two strings, i.e. in this case between open G-string and fourth position on the C-string. It's possible to achieve an extremely fast pizz tremolo this way by using the following pattern:
- Thumb on C-string
- Index finger on G-string
- Index finger on C-string. This is not a separate movement: the finger comes from the G-string, drops right onto the C-string, which it then immediately plucks without further preparation. Bassists call this technique raking.
- Middle finger on the G-string.
That's really an usual approach though – you'll definitely want to lay down your bow for something like that. And it's really hard to get an even sound despite alternating between C and G string. I'm very sure Holst didn't intend the passage to be played like that – but it has the potential to yield by far a denser, louder sound then either of the other options.