Using the Telltales
The mainsheet trimmer should be constantly checking the telltales on the leech of the sail to make sure that they are flying correctly. The aim is to have the mainsail at the angle of attack that maximises lift without stalling the sail.
To do this the trimmer will bring the mainsheet in until the top telltale stops flying constantly and starts to dip forwards around the leeward side of the sail. The telltale does this when the airflow on the leeward side of the sail falls off the sail before it gets to the leech. Air from the windward side of the sail then flows around the leech and heads forwards along the leeward side of the sail. To stop the sail from stalling like this the trimmer will let the sail out slightly.
The trimmer constantly trims the mainsheet so that the top telltale is flying about a third of the time. All the other telltales on the leech should be flying constantly (if they are not then there is a problem with the shape of the mainsail).
Avoiding Weather Helm
When heading to windward in heavier winds the mainsheet trimmer has a vital role in the trim of the boat. It is very important that the yacht sails as flat as possible because this reduces leeway and increases lift. When all available crew are up on the windward rail and there are no extra bodies to help trim the boat the trimmer will need to start dumping some mainsheet and/or traveller in order to minimise weather helm. He or she will constantly be playing the gusts, letting it out when the boat heels and bringing it in when the boat levels. Good communication with the driver is important in these conditions because the driver will want to take the opportunity to feather the boat into the gusts and point higher for several seconds. If the helm is doing this then the mainsheet trimmer will not need to spill as much power from the mainsail.
As the boat comes through a tack the tendency is for the trimmer so sheet the main in hard (especially in heavy weather as it saves having to strain to bring the main in when it is under a full load). The trimmer should avoid this tendency and ease the main for a few moments to allow the yacht to come up to speed before trimming the sail.
Another function of the mainsheet is to control the amount of twist in the mainsail. When the mainsheet is eased twist will increase.
Before setting out it is useful for the trimmer to apply PFE spray to the traveller and mainsheet blocks.