The mainsheet traveler is used to optimize the width of the slot between the mainsail and the jib. By precise adjustment of the traveler, you can ensure that the slot between the mainsail and jib is neither too wide nor too narrow to accept the volume of air that is being directed through it. The width of the slot must be correctly adjusted in order for the sails to generate their maximum amount of power. You should adjust the jib correctly for the course that you will be sailing, and then adjust the mainsail until the jib just begins to backwind and lift the luff of the mainsail. When that happens, you know that the width of the slot is just right.
When the windspeed increases, and the boat becomes overpowered, you can narrow the slot and reduce the amount of power that is generated by the sails by easing the traveler to leeward. You can also narrow the slot and reduce the amount of power by easing the mainsheet, but that changes the shape of the mainsail in ways that are not helpful. By correctly adjusting the relationship between the mainsail and the jib, you also maximize the boat''s ability to point.
Also, when you bear off the wind from a beat to a close reach and you ease the jibsheet, you are opening up the slot between the mainsail and the jib. By adjusting the traveler to leeward, you can optimize the width of the slot for that course. As you bear off even further (beyond the range of adjustment of the traveler), you need to ease the mainsheet to adjust the width of the slot, and you use the boom vang to control the shape of the mainsail.
If your sails are badly trimmed, and you suddenly correct that condition, then the difference will be enough to register on your knotmeter. But, if your sails are basically trimmed correctly, and you are making the kind of fine adjustments that a skilled racer would use, those fine adjustments can''t be measured by your knotmeter. Your knotmeter isn''t accurate enough to register differences in the range of an umpteenth of a knot. You have to read the authorities on sail trim, learn the basic principles, and take it on faith that, if you follow their advice, the boat will go minutely faster and point minutely higher. The only way that you can know for sure those techniques are working is by comparing your progress with the progress of other boats or racers, to see whether you are consistently gaining on them or falling behind.
Moving the traveler to windward and easing the mainsheet in light air is only helpful so long as the width of the slot is optimized. If it widens the slot, it could be detrimental. Your first concern should be to shape your sails as full as possible and to get the width of the slot right. Then you can make slight adjustments to the mainsail shape to power up the mainsail to the nth degree. But don''t expect to see or feel any resulting increase in speed.
In my opinion, there isn''t any difference between the basic techniques that a skilled racer and a skilled cruiser would use. The same sail trimming techniques that help a racer drive efficiently to the windward mark also help a cruiser claw off a lee shore. The same techniques that get a racer around the course ahead of the competition will get a passagemaker to the destination three days ahead of schedule. You don''t have to learn all the sophisticated sail trim techniques to enjoy sailing, but the more you learn, the more efficient and safer sailor you will be.