what does this do? front or rear?

one-way is for the front. it aids in 4wd front steering. on-power, the front pulls just like normal, but off-power, it lets the front wheels spin free of power just like a 2wd would.

does it make the front a locked diff?

It makes the front end locked when on the power.the front end freewheels on decel.will have no braking in frt.

Basically giving you better cornering and faster acceleration, but at the expense of braking and front end stability. A one-way works on most tracks that you can coast or use small brake inputs to make the corners.