  The Hamiltonian representation of magnetic field lines enables one to study
their confinement properties in tokamaks through the use of symplectic maps
such as the symmetric tokamap and its bounded version, the latter being
introduced here. In this time-independent purely magnetic framework, we
observed the drastic improvement in the confinement of magnetic field lines
produced by the local vanishing of the shear profile. This amounts to a
non-twist condition that notably acts in the same way the safety profile being
(non-strictly) monotonic or having a reversed shear. We single out the effect
of the amount of flatness of the safety profile in the vicinity of its zero
shear point. All other things being equal, the beneficial effect of the
vanishing of the shear profile is shown to be increased if the radial extent of
the low-shear region is increased. To be specific, the low-shear region induces
the formation of a belt of robust KAM tori acting as an internal transport
barrier whose width is all the larger as the extent of the low-shear region is
broad.
