  Quasi-periodic oscillations of the hard X-ray (HXR) emission of the large
flare of 2 November 1991 have been investigated using HXR light curves and soft
X-ray and HXR images recorded by the {\sl Yohkoh} X-ray telescopes. The results
of the analysis of these observations are the following: i) The observations
confirm that electrons are accelerated in oscillating magnetic traps which are
contained within the cusp magnetic structure. ii) The amplitude of the HXR
pulses increase due to the increase in the amplitude of the magnetic trap
oscillations and the increase in the density within the traps caused by the
chromospheric evaporation upflow. iii) The increase in the amplitude of the HXR
pulses terminates when further increase in the density inside the traps
inhibits the acceleration of electrons. iv) The model of oscillating magnetic
traps is able to explain time variation of the electron precipitation, strong
asymmetry in precipitation of accelerated electrons, and systematic differences
in the precipitation of 15 and 25 keV electrons. v) We have obtained a direct
observational evidence that strong HXR pulses are the result of the inflow of
dense plasma coming from the chromospheric evaporation, into the acceleration
volume.
