Just WATCHED a truck roll over

[Wayne64SS]

@toomanycamaros

not even close. A Jake physically keeps both intake and exhaust valves closed when your not on the throttle, therefore providing engine compression base braking. The exhaust butterfly style brakes close off the exhaust creating increased back pressure and kind of doing the same thing, but not nearly as much engine braking force.

The jacobs brake opens the exhaust valve at the top of the compression stroke.