On a medium sized boat, 35 to 45 feet, I like to use three people to tack the sail: one to release the old sheet, one to tail the new sheet, and one to grind and do the final trim. When a tack is called, the releaser is the first one off the rail. He or she goes to leeward, uncleats the jib sheet, and unwinds a few wraps off the winch, being careful not to let the sheet slip. There should just enough wraps left on the winch to hold the sheet. As the boat turns into the tack, the releaser watches the bow of the boat and the luff of the jib. As the luff starts to backwind, an ease of about 3 to 4 inches will help the boat turn and take a bit of the load off the sheet. Once the jib is backwinded about halfway to the mast, release the sail completely by taking off all wraps. On this size boat you can usually lift the sheet vertically and pull all of the wraps off the winch. If it's windy and you're tacking a genoa, you may need to make the release a bit more controlled by taking the wraps off and letting the sheet run through your hand.