Matt, I disagree.
They read off the first curtain and the film. It does no good to read off the second curtain because its traverse speed cannot be regulated. Once the second curtain is released the exposure will end when it completes its traverse. So exposure timing ends with release of the second curtain (its traverse time is of course figured in to the total timing).
Other designs decide on the exposure at the moment the mirror swings up, freezing the exposure at that point. The OTF timing continues until the moment the second curtain is released. It never stores an exposure time, but relies only on total accumulated light. If an intense light source hit the subject at the instant the second curtain was released, there would be no way to correct for it.
OTF shutter timing is still more effective at higher speeds because the metering system remains active until release of the second curtain, instead of release of the mirror.