No Light place

Lock engaged - offhand torch won't place

With the lock toggled on, right-clicking no longer dumps torches from the offhand.

Lock disengaged — offhand torch places as normal

The same tunnel, one key-press later. With the lock toggled off, the offhand goes back to plain vanilla behaviour — right-click drops the torch down exactly as you'd expect.

Exemple (left hand off)

Exemple (standard work)