Without knowing the full details of that small scale operation there could be a number of reasons why they detect the wash first.
My opinion is that seeing the 2.5kg nugget & other large specci on the vid they don't want the trommel to screen large pieces of gold out. A trommel helps break up & classifies the gravels/clay/dirt etc. into smaller material for the sluice. Larger material exits out the back. It's possible they have found that their larger bits are/were/could be classified out the back?
The video also shows that the larger nuggets are found predominately on one side of the cut. They may only be pre detecting this material to accurately find the larger pieces first prior to washing.
It could also be that water is an issue on that site? Washing might only be possible when water levels dictate? They may also need to stockpile a certain amount of wash dirt in order to operate the wash plant efficiently i.e. have enough dirt to feed it for a period then clean up for profitable return? If water &/or stockpiling is the issue it may be just more efficient for them to pre detect all the wash in order to maintain enough working capital for continued operations? Detecting it could enable them to quickly & accurately extract any reasonably large bits for immediate sale/revenue?
