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In our recent paper (Solar Physics 261, 233) we investigated quasi-periodicoscillations of hard X-rays during impulsive phase of solar flares. We havecome to conclusion that they are caused by magnetosonic oscillations ofmagnetic traps within the volume of hard-X-ray (HXR) loop-top sources. In thepresent paper we investigate four flares which show clear quasi-periodicsequences of HXR pulses. We also describe our phenomenological model ofoscillating magnetic traps to show that it can explain observed properties ofHXR oscillations. Main results are the following: 1. We have found thatlow-amplitude quasi-periodic oscillations occur before impulsive phase of someflares. 2. We have found that quasi-period of the oscillations can change insome flares. We interpret this as being due to changes of the length ofoscillating magnetic traps. 3. During impulsive phase a significant part of theenergy of accelerated (non-thermal) electrons is deposited within the HXRloop-top source. 4. Our analysis suggests that quick development of impulsivephase is due to feedback between pulses of the pressure of acceleratedelectrons and the amplitude of magnetic-trap oscillation. 5. We have alsodetermined electron number density and magnetic filed strength for HXR loop-topsources of several flares. The values fall within the limits of $N \approx (2-15) \times 10^{10}$ cm$^{-3}$, $B \approx (45 - 130)$ gauss.

Investigation of quasi-periodic variations in hard X-rays of solar flares. II. Further investigation of oscillating magnetic traps