Complex sentence: If the running backstays leads to the mast where the forestay attaches, the effect of tensioning them will reduce sag in the headsail and second, increase mast bend and flatten the mainsail as a result.
Split into simpler sentences: If the running backstays leads to the mast where the forestay attaches, the effect of tensioning them will be as follows. Again the forestay is tensioned reducing sag in the headsail and second, mast bend is increased with flattening of the mainsail as a result.