Fit_one_cycle -> pct_start param?

I wonder how and when to user the ‘pct_start’ parameter of the function ‘fit_one_cycle’. I digged down into the docs and source code until the ‘OneCycleScheduler’ class which uses this parameter for learning rate annealing:

 def on_train_begin(self, n_epochs:int, epoch:int, **kwargs:Any)->None:
        "Initialize our optimization params based on our annealing schedule."
        res = {'epoch':self.start_epoch} if self.start_epoch is not None else None
        self.start_epoch = ifnone(self.start_epoch, epoch)
        self.tot_epochs = ifnone(self.tot_epochs, n_epochs)
        n = len( * self.tot_epochs
        a1 = int(n * self.pct_start)
        a2 = n-a1
        self.phases = ((a1, annealing_cos), (a2, annealing_cos))
        low_lr = self.lr_max/self.div_factor
        self.lr_scheds = self.steps((low_lr, self.lr_max), (self.lr_max, self.lr_max/self.final_div))
        self.mom_scheds = self.steps(self.moms, (self.moms[1], self.moms[0]))
        self.opt = self.learn.opt, = self.lr_scheds[0].start,self.mom_scheds[0].start
        self.idx_s = 0
        return res

But I cannot figure out what this exactly means? Any help appreciated…