Witnesses going back and forth (4)
I believe the answer is:
sees
I believe this is a double definition.
'witnesses' is the first definition.
(I know this)
'going back and forth' is the second definition.
(Other definitions for sees that I've seen before include "Watches" , "Looks (both ways!)" , "Observes or perceives" , "Bishops' dioceses" , "Spots" .)