A man is riding a horse while the horse in bucking.
Man riding a horse
We assume the man is the same in both sentences and he's riding a horse.
exists e1 x1 x2 x3.(in(e1,x3) & n_bucking(x3) & while(e1,x2) & n_horse(x2) & n_horse(x2) & Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_man(x1))
exists e1 x1 x2 x3.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_horse(x3) & (x1 = x2) & n_man(x1))