UCD3138: [TP PFC]TP PFC control at zero crossing questions

Hi everyone,

I got 1kw UCD3138 totem pole pfc evm,it's workable then i found someting interesting ,only one of dpwm would have some pulse at ac zero crossing, it just like softstart for getting a good ITHD ,am i right?

does this control method have some detailed description or documents?

thanks.