It's a programming term from long ago. When you wanted to activate some obscure and not well (or at all) documented feature, you "poked" a value into a memory location. C64, Apple][, Trash 80, that era. I'm showing my age. 
I get that. I'm just trying to find out what the intended behaviour and current state is right now. Some places suggest it's a test-only feature, some suggest it's doable right from LuCI, no "pokes" required. Even turning it on manually I can still only get it to only partially work, and I'm not sure if I'm not doing things right, if it's my hardware (which is a bit of a kitbash), or the feature is not fully implemented.
Not asking anyone to solve my issues. But I would like to know if anyone else has it working even partially, and if they do to maybe start collecting some configs to see what the common denominator is. Then I can write up something in the Wiki to help document where the feature is at.
To that end...
What works (for me):
- One "radio" in STA mode connecting as a client to another router
- The other "radio" in AP mode successfully creates an SSID that devices can at least see
Issues:
- Activating the feature (with
echo 1 > /sys/kernel/debug/ieee80211/phy0/mt76/dbdc) returns an error, though it does appear to activate the second PHY - Devices can connect to the AP radio, and are successfully associated, but then immediately dissociated.
- Both PHY0 and PHY1 appear to support both 2.4G and 5G, which for the 7615 is definitely not the case.
- Device lights not working properly.
Looking for anyone else using the feature in OpenWrt and what works for you