What does frequency foldback do in a switching regulator?
Frequency foldback as shown above is simply reducing the switching frequency.
If the switcher is running at the minimum ON time but the inductor current is still rising (the inductor will actually discharge more slowly if there is a heavy load), then the only option left is to reduce the frequency.   

You can see on the waveforms when the output voltage drops to 0V, the switching waveform (Vsw) reduces it's frequency. Also note the discharge slope of the inductor is more shallow (bottom waveform)