The weight-jacker is a setting somewhat akin to wedge on a stock car. The main difference is that the driver can adjust weight-jacker while on the track. There are even rumors of some drivers adjusting weight-jacker in between corners to get the most out of a lap.
Increasing weight-jacker makes the car looser. The weight-jacker goes from -15 to 15. Bear in mind that the setup keeps the last weight-jacker settings and can cause a setup to fail tech in the garage.
I try to get the car setup to where it’s a smidge free with the weight-jacker set to 0. When I leave the pits or at race start, I’ll set it to -1 to add a little push to the car for the first lap while the tires are cold.
Important to note that just 1 click of weight-jacker can have drastic effects on the amount of over/ understeer in the car. It’s a brave soul who will click it more than once at a time.