The fourth essential step in winemaking is to siphon the wine off the sediments (lees) into another clean secondary, reattach the fermentation trap, and repeat after another one or two months and again before bottling.
This procedure is called racking. It is done when necessary, not just two or three times as stated above. The rule is, as long as there are fresh deposits on the bottom after a regular interval (30 to 60 days), even if they are just a light dusting, the wine should be racked. Only when that interval passes and there are no fresh lees -- AND the specific gravity is 1.000 or lower -- is the wine ready to be prepared for bottling.
It is not necessary that the interval between rackings be 30 days, 45 days or 60 days, but it should not be less than three weeks. It is perfectly okay to leave the wine on the lees for three months. Beyond that and the wine enters a danger zone caused by dead yeast cells breaking down -- rotting. While this can cause off-flavors and odors if allowed to go on too long, the bigger danger is the formation of hydrogen-sulfide gas, which smells like rotten eggs and can be the death of the wine. But if the lees are stirred every week or so, neither the off flavors, off odors nor hydrogen-sulfide gas form. Indeed, the wine is actually improved by extended contact with the lees as long as they are stirred frequently.
During this entire period, the sulfur dioxide gas released into the wine from crushed Campden tablets or potassium metabisulfite is slowly dissipated through the airlock and its protective qualities are lost. It is therefore necessary to replenish this protection, and this is done at every other racking. The new sulfites are added to the receiving secondary and the wine racked into it.
As in the transfer of the wine from the primary to the secondary, during rackings the wine's exposure to oxygen-lade air should be minimized. This is much easier to do during racking than during the initial transfer, as the downward end of the racking tube can easily be directed against the inside wall of the carboy or under the surface of the transferred wine. Again, those who are extra cautious can sparge the receiving carboy with carbon dioxide or argon gas before racking the wine into it.
Racking can be made easier by attaching a racking wand to the racking hose. A racking wand is a rigid, L-shaped plastic tube that allows you to better control where the bottom of the take-up is located. The take-up end of the wand is fitted with a protective cover that allows the wine to enter the wand from above rather than below. This helps prevent it from sucking the lees below it into the receiving secondary. A racking clip can also be attached to the mouth of the carboy to hold the take-up end of the wand at a determined height (or depth). This depth should be midway between the surface of the wine and the lees, and adjusted periodically to maintain that aspect.
Racking is not as difficult as many new winemakers make it. There is no reason to agonize over racking at an exact interval, or leaving the wine in contact with the lees an extra week -- or even a month. But is is prudent not to be sloppy about it, and to sanitize all equipment with sulfited water or a specialized sanitizer before and after use. Cleanliness in winemaking is everything.