1. Trimming the backstay….
a) tightens the leech of the main
b) moves the draft of the jib aft
c) makes you point higher
d) flattens the main
The backstay tightens the rig and depowers both the main and the jib.
a) The tip of the mast is pulled aft and down which opens up the leech of the main so it twists more near the top allowing more air to spill off; with less pressure higher up, the boat heels less. Tightening the backstay also bends the middle section of the mast forward which pulls out and flattens the belly of the main.
b) Tightening the backstay puts more tension on the forestay and the jib halyard. When you tighten the luff of a sail you move the draft, or position of greatest depth, forward. In heavier wind you want the draft of the jib to be farther forward so it forms a sharper knuckle to part the wind.
c) Going upwind, a properly trimmed jib and main play a balancing act with their pivot point on the keel. Pressure on the luff of the jib and on the leech of the main counteract one another to produce a neutral helm (or whatever is fastest for the boat, usually a little bit of weather helm but sometimes lee helm, as is the case with a J/24). When you tighten the leech of the main it pivots the boat into the wind allowing you to point higher so opening up the leech by tightening the backstay does not make you point higher; it reduces heel and helps you sail faster.
Here is a video taken by Chris Poole of the main going from full backstay on to no backstay.