The 1.9 thread is basically a way to level up your Great Building by borrowing forge points from people who have a level 80 Arc. A level 80 Arc pays 90% more for contribution rewards. So they can give 1.9 times the reward and they will get all of their forge points back when the Great Building levels. The term “locked” means the position cannot be stolen from the person as long as the pay 1.9 times the reward. It is important when the person lending you forge points puts their forge points on, they are locked. If not, the position could be stolen costing them thousands of forge points.
Here is the form to calculate locking in a position: