That's basically all the FCOM says. If the bank angle is greater than 30º it will command a roll back to 30º, otherwise maintain the bank angle and V/S that existed at the time the F/D was switched on. So, when you do that while the PF is flying a visual approach, the flight director indications very quickly become irrelevant as it's trying to maintain what was happening at that instant. As Good Cat said, the purpose of turning the PM's F/D back on is simply so that the (useful) indications are not lost during a missed approach when the TO/GA mode ends. In most other situations, immediately after turning on the F/D you would make another selection and not remain in ATT.