a rollover is just that, where u sell and buy back virtually at the same time.
It works like this , u buy shares on credit on a t(+) basis where the + = no of days - anythign from t3-t25. Lets say u buy shares on a t25 basis and in 23 days u have to decide whether to sell, to buy outright or to rollover ur shares for the same period. This means u have another 23 days left to decide what to do and hopefully it will rise.
the very small difference in spread and the postign of buy and sell amount of same volume at same time is the giveaway(tho advfn software usually puts them both in same column)