at first, the BJT is switch on ,the current from Vin to L1\ BJT to ground, L1 voltage up+ down-,the L2 Voltage are up- down+,D can not conductor ,L2 no current, the transformer L1//L2 like a inductor only L1, the increase current flow L1 is Vin*ton/L1,the energy stored in the L1 or transformer;
the second , the BJT is switch off, at the time of BJT switch off,the current flow in L1 are not down to zero immediately, it charge the capacitor of the BJT Cce , when the voltage of Cce up to Vin+N*(Vo+Vf)(N is the rate of L1/L2 circle number,Vf is the D forward voltage),then the L1 voltage is up- down+,the L2 voltage is up+ down- and equal to (Vo+Vf),then the D is conduction,the current store in L1 are transmit to L2 and output to C and road,the energy stores in L1 or transformer are send to output.
when the BJT switch on and switch off cycle by cycle ,the energy can transmit to output from input,change the switch on time of one cycle(duty cycle control or PWM control),the energy be output can be controld.