The halyard tension alters the draft position dramatically. Its disadvantage is that it can be difficult to control easily in a lot of wind and is often tolerated! A drop in halyard tension, also drops the clew, so the twist is increased, and less twist if it is tensioned up. Be careful with this, and re-trim the sheet after any change of halyard tension.
The sheet controls the clew height, forward/aft position and twist, in three planes. The horizontal plane – the sail in or out – is the obvious one, but it also lets the clew up, and outboard and twists the headsail – a more “open” leech.
Some twist is good trimming, and the use of the telltales lets you know the correct twist.
In-haulers – only on Jib rigged boats, it is a very powerful way to get the clew inboard, and acts for the jib like the traveller for the main. But the clew is being moved around a lot, and the rules on twist and camber must still be carefully applied.
Using the backstay tensioner is the most effective tool to change the shape of the Headsail when sailing, as it alters the headstay sag, and therefore the fullness of the sail. On boats with fractional rigs with swept back spreaders, the mast will be bent to some extent all the time – pre-bend. This is the way that these masts set up properly and securely, but the backstay gives more headstay tension, and so still affects the shape of the headsail.