  We report on the intense burst ``forest'' recorded on 2006 March 29 which
lasted for ~30s. More than 40 bursts were detected both by BAT and by XRT,
seven of which are rare intermediate flares (IFs): several times 10^{42} ergs
were released. The BAT data were used to carry out time-resolved spectroscopy
in the 14-100keV range down to 8ms timescales.
  This unique dataset allowed us to test the magnetar model predictions such as
the magnetically trapped fireball and the twisted magnetosphere over an
unprecedented range of fluxes and with large statistics (in terms of both
photons and IFs). We confirmed that a two blackbody component fits adequately
the time-resolved and integrated spectra of IFs. However, Comptonization models
give comparable good reduced chi^2. Moreover, we found: i) a change of
behavior, around ~10^{41} erg/s, above which the softer blackbody shows a sort
of saturation while the harder one still grows to a few times 10^{41} erg/s;
ii) a rather sharp correlation between temperature and radii of the blackbodies
(R^2 prop kT^{-3}), which holds for the most luminous parts of the flares
(approximately for L_{tot} > 10^{41} erg/s). Within the magnetar model, the
majority of these findings can be accounted for in terms of thermalised
emission from the E-mode and O-mode photospheres. Interestingly, the maximum
observed luminosity coming from a region of ~15km matches the magnetic
Eddington luminosity at the same radius, for a surface dipole field of ~8 x
10^{14} G (virtually equal to the one deduced from the spindown of SGR
1900+14).
