The behaviour is documented in the field and has been observed repeatedly in captivity. A turtle plants itself on soft ground and alternately strikes with the forefeet or drops the front of the shell, producing a rhythmic vibration. Within a minute or two earthworms emerge at the surface nearby and are eaten.
The likely explanation lies in the worms' own defences. Earthworms flee to the surface from vibrations resembling a digging mole, an escape response that human worm charmers exploit with stakes and saws. The wood turtle appears to have arrived at the same exploit independently.
A related behaviour is sometimes described in gulls and other birds, so the wood turtle is not unique in using it, but it is the only turtle for which such vibration foraging is well described.