A simple Bang-Bang or Hysteretic SMPS as discussed in:
The nominally 5V SMPS output is loaded with a fixed resistive 5R load (so about 500mA) in parallel with a swept sinewave current sink of +/-250mA.
The swept sink starts at zero current until after the SMPS output has reached a steady state and then is swept in frequency from DC to above the natural (steady state) switching frequency of the SMPS. This is at about 80kHz.
Careful examination of the switching frequency as indicated by V(SW) between about 1.4ms and 1.5ms will show that, as the frequency of the ac load current from the current source B1 is swept, the actual switching frequency becomes synchronised to the swept load frequency for a range of frequencies around the 80kHz natural switching frequency as observed with no ac load current component.
This phenomenon of frequency locking is called "entrainment" and is discussed in more detail here:
Entrainment is a feature particularly of hysteretic power supply converters and so can be observed in most SMPS topologies including Boost, Flyback and Forward converters. Links to learning resources and information about switched mode power supplies:
I can recommend the books from here:
There are some useful articles on his site and some more here:
but the books are worth every penny.