Reefing is a way of reducing the area of a sail by folding one edge of the canvas in on itself. The loose material is folded, and lashed down to the boom with sail ties. Carrying a smaller sail is a safety precaution, reducing heeling and de-powering the sail to improve safety and stability in rough weather.
When sailing, if you can do the same speed with less sail, do it. As soon as you think of reefing down, do it. Should we reduce sails? Yes, always yes.
Shaking out a reef is easier than putting one in in high winds.
Before we start our night shifts, we usually reef the mainsail. A smaller sail will slow us down, but our main concern is safety. The rule is that if the wind rises enough to have us question whether we should reef or not, we do it. Another important rule, is to never reef at night. Everything is harder to do in the dark. We have done it before, with success, but the issue is that it means waking up your partner, robbing them of their precious sleep. Can't we reef a sail alone? Yes, we could, but that would break another rule: never wander on deck in the dark alone, even with a tether. Therefore, we plan to always do our last manoeuvres of the day while there is still some light out, at the cost of speed.