A blocking oscillator has the oscillation feedback path through a
resistor and capacitor in parallel.  During the half-cycle where the
transistor conducts, positive feedbck is through the capacitor.  Once the
capacitor has become excessively charged to maintain enough feedback to
keep the transistor saturated, this half-cycle ends.  This half-cycle is
usually the shorter one.  During the other half-cycle, the transistor is
off until the capacitor is discharged sufficiently by its paralleled
resistor to allow current to flow through the base of the transistor.

  In a ringing choke oscillator, the oscillation feedback (assuming a
bipolar transistor) is through a resistor.  The "transistor-on" half cycle
is usually the longer one.  That half-cycle ends when either the tapped
inductor saturates or the transistor comes out of saturation.  The
transistor-off half cycle's onset reinforces itself with the feedback
winding's voltage changing in a direction to reduce the transistor's
collector current.  In fact, once collector current starts decreasing, the
transistor usually quickly slams off, and a high voltage pulse can occur.
  If a ringing choke oscillator is not carefully designed, the transistor
may be destroyed by breakdown from high voltage pulses resulting from
suddenly interrupting current flowing through the inductor.
  The transistor-off half cycle ends when the transistor's collector
current has decreased to zero and has become steadily zero.  There may be
a delay for stray capacitance charged by the high voltage pulse to
discharge before the feedback winding produces voltage in the forward bias
direction, but that is usually short.

 - Don Klipstein (d...@misty.com)