From the reflection of the sunlight! Light bounces around everything and everywhere, so while it's not direct sunlight, it's still light. It generates less than the sun facing one but it's still more power.
Is it worth the additional cost tho? I guess if you’re limited on space there is no other option, but I feel like I’d rather entertain additional solar panels that are facing the sun than to add additional cost for such a low return.
Alternatively, you lay them out in north-south rows (like a fence) and they generate most power during morning and evening, when sunshine comes from east and west.
Coincidentally, their power maximum is when conventional solar parks are not yet producing or no longer producing.
Coincidentally, this also matches the ideal agrivoltaic setup, where you use almost zero surface (the panels are vertical) and grow plants between, not below solar panels.
Wouldn't it be a more efficient use of space to just put up another solar panel facing the sun instead? No need to elevate just to reflect the light into the underside.
Bifacial panels have been in use for several years in utility-scale solar installations because its cheap enough to do that the modest additional electricity generated is worthwhile.