Slats and flaps are high lift devices (that asume you have good knowledge of) that you use when you have a high angle of attack, normaly during take-off and landing. If you would have the slats and flaps out when you fly with low angle of attack, which is the case during normal fligh, you would add drag. The drag must be quite high since all planes remove them during normal flight.

Now back to sailing!
A sloop always sail with a bit of flaps and slats out on the upwind where you have low angle of attack, but the uni can remove them completly during the upwind by taking down the spi. On the downwind where you have high angle of attack both the sloop and uni pulls out the high lift devices (the spi)

/hakan