Backscattered electrons (from the original beam) retain much of their high energy (typically kilo-electron volts). Secondary electrons tend to be very low energy (tens of electron volts).
They can thus be sorted by putting a slight negative bias on the upper detector which is enough to impede the lower energy secondary electrons.
The unbiased lower detector collects all electrons reaching it. However, the larger number of secondary electrons swamps out the contribution from the backscattered electrons.
