Mark,

When you use a vernier scale, you read the fraction of the second off of the appendage to the second hand by identifying which hash mark lines up exactly with a dial marking on the outer register. Only one hasmark should line up exactly. Thus, if you stop the chrono hand at or after 10 seconds, and the line on the vernier scale at the number "3" lines up with a hashmark, that's 10.3 seconds. This is what makes a separate hand unnecessary. It's a clever - if a bit egghead - solution. Jeff's question is still a darned good one, because of the beat rate of the v72 - only a 1/5 second accuracy should be possible.
LS

: I know some Vj 72s are modified to run at 21600 bph, the Rolex 727
: being a prime example, but I suspect we'd have heard more about
: it if anyone had got one running as fast as an el Primero! The
: components weren't designed to run twice as fast as
: "normal" speed so the amount of revision required
: would have resulted in a new movement really.

: And how would that scale work, without a separate hand to run off?
: The chronograph seconds hand will take 60 seconds to rotate once
: round the dial, so that makes the additional scale pointless and
: redundant from what I can see. So it looks to me as if it's just
: an addition for "style" as you surmise, in which case
: I find it a bit misguided, as that garish orange scale detracts
: from what would otherwise be quite a handsome dial (though
: better if the registers were equally sized and finished the
: same).

: Maybe someone can put me straight, but I just can't see how it
: would be functional...