Wary traders let many of their bets on the Nifty index lapse on expiry of the April derivative contracts but rolled over their positions in select volatile stocks to the May series. Analysts said traders have opted for prudence above valour ahead of an election outcome that in many ways promises to be a watershed event.
The futures and options (F&O) rollover gauges market sentiment as bets in this segment reflect traders' and investors' expectations of how it would react to certain events.

Source - http://economictimes.indiatimes.com