Your miner is set to mine SIGT, pool receives shares for SIGT, you get paid SIGT in the background, pool bot continuously moves your earnings (let's say accumulated earnings every 15min) to supported exchange, automatically sell SIGT/buy BTC and nominate transacted BTC to your account. Now the SIGT/BTC rate for that particular transaction may not be the most favourable. I assume this is what you were asking...
What has been pointed out is you accumulate SIGT, then move SIGT to exchange and make the transaction yourself when candle chart has some nice green bars in the higher ranges. Though, writing about it is always easier than applying the theory in real situations.