Look at the orbits of the constrained subspace under the action of the symplectic flows generated by the f 's. This gives a local foliation of the subspace because it satisfies integrability conditions (Frobenius theorem). It turns out if we start with two different points on a same orbit on the constrained subspace and evolve both of them under two different Hamiltonians, respectively, which agree on the constrained subspace, then the time evolution of both points under their respective Hamiltonian flows will always lie in the same orbit at equal times. It also turns out if we have two smooth functions A 1 and B 1, which are constant over orbits at least on the constrained subspace (i.e. physical observables) (i.e. {A 1,f}={B 1,f}=0 over the constrained subspace)and another two A 2 and B 2, which are also constant over orbits such that A 1 and B 1 agrees with A 2 and B 2 respectively over the restrained subspace, then their Poisson brackets {A 1, B 1 } and {A 2, B 2 } are also constant over orbits and agree over the constrained subspace.