They shift the position of the eyelets in the shock relative to the frame. In the case of the Jekyll and Trigger, they make the frame see the shock as a little bit longer than it actually is. On a pull shock this has the same affect on the bike as compressing the shock slightly would, making the bike sit lower at the start of the travel. This does two things, it lowers the bottom bracket and slackers the head angle. One bush moves it about 0.75 degrees, two 1.5. Each bushing drops the BB around 5mm.
Stock, your bike has a 68 degree head angle in elevate. When you flick the lever to flow it slackens to 67.5 and the BB drops 5mm.
If you stick one bush in, you will start at about 67.2 or thereabouts, and the drop to around 66.7 when you move to flow. If you run with two then you’ll start around 66.5 in elevate and drop to 66 in flow.
Modern trail bikes tend to be around 66.5 to 67. The Bronson for example is 67. More burly enduros like the Yeti SB6 are around 66. I do a lot of climbing so going below 67 in elevate seems too much, not that I’ve tried.
Slacker should mean more stable at speed downhill, and easier to lift the front. Small changes apparently gave a big impact. I’ve not tried one of these yet. Too slack and it wanders and lifts when climbing.