Judicial restraint in the mold of Frankfurter takes three forms. In the first place, a justice like Frankfurter grants wide latitude to legislators and government officials. There is not necessarily a faith involved that politicians will do the right thing, but there is certainly an attitude that they are better left to sort out their mistakes for themselves.
Another form of restraint is strict adherence to precedent. For all justices, the Court's past decisions guide their current jurisprudence. But here, that deference is often granted even when the Court's earlier decisions are acknowledged to have been in error. Finality trumps correctness.
Finally, there is deference specifically to state laws and officials. This is the principle of federalism. The key issue here by the time of Gideon's case was how much the Constitution limited the powers of state governments in light of the Fourteenth Amendment. For many decades, the Court declined to use either the amendment at all to impose the restraints of the Bill of Rights on the states. Since a 1925 case which upheld the right to free speech against state interference, however, the Court gradually conceded that some rights are so fundamental that they must also apply to state governments. Such rights were held to have been absorbed into the amednment's due process clause. The Court's advocates of restraint, however, were cautious about which rights could be absorbed.
For his part, Black was not satisified with the doctrine of absorption. He saw no firm principle to distinguish fundamental rights which should be absorbed from the others which should not. He saw too much latitude there for justices to make arbitrary decisions, and favored instead a simple, wholesale incorporation of the first eight amendments into the Fourteenth Amendment.
I don't doubt that Frankfurter is one of the great lights of the Court's history, but I can't muster much respect for his position here. Whatever it's uses for easing the path to enforcing Constitutional rights, the doctrine of absorption strikes me as nonsense on steroids. The privileges and immunites clause plainly applied the existing federal rights of citizens against the states, and if these rights were not those enumerated in the Bill of Rights then there really wasn't anything else for them to be.