shifting gears down - but following which rule?

[Helmuth]

Guys,

I am going downhills really often and long and sometimes she helps me avoiding to step on the break because she shifts one gear down - nice behavior !!

BUT:

I have never found out when she does that. Like an iPhone - great app but nobody knows how to use it. Tried to use the break several times sometimes she shifts down - see that on the RPM - but not always .... ??

Any advice or hint?

Sorry for not beeing an english native speaker.

Best regards,

Helmuth