Undocumented ("Hidden") Feature:  Active Roll Axis Cockpit View (DriverHeadHorizon)

There is a neat hidden feature called DriverHeadHorizon that can be enabled via the iRacing app.ini file which enables a "roll" effect that helps you to visually observe the amount of roll the car is experiencing as it goes over curbs, or as the banking of the track changes from flat to banked.

This view effectively locks your in-car view to the horizon, while allowing the cockpit of the car to bank to the left and right in response to banking changes or roll inputs -- the effect is like following an airplane in a flight sim in chase view with the camera view (or your virtual "eyes" in the game) "locked" to the horizon, and the plane that you are following free to rotate around its roll axis.  In this scenario, the horizon would remain horizontal at all times, but you would see the airplance's wings tilt to the left and the right as it banked left and right.

In iRacing, when you enable the DriverHeadHorizon feature, the camera point of view (or your virtual "eyes" in the game) will remain locked to the horizon and the horizon will remain perfectly horizontal, but the cockpit around you will become more active, and you will effectively be able to see the car's dashboard banking slightly to the left and the right in response to changes in roll and banking of the track, much like the wings of the plane in the flight simulator example above.  The effect in iRacing is fairly subtle, and many people feel that this makes the driving experience infinitely more immersive immediately, although some people with more sensitive inner ears (and thus more prone to motion sickness) report that this feature can have a negative effect.

Most people who enable the DriverHeadHorizon feature report that they can "feel" and "read" the track better because they gain a visual indication of the roll attitude of the car with regard to the banking of the racing surface, curbs, etc., thus improving the overall driving experience.

In order to enable this function, you will need to edit the app.ini file -- just find the following value under the [View] section and set it to "1" to enable the DriverHeadHorizon feature: 

DriverHeadHorizon=1.000000     ; Percent to allow the drivers head to stay level with the horizon when the car tilts.

Note that if the effect seems to extreme to you, this value is adjustable from "0" to "1" -- at "0" the effect is completely disabled (or 0% roll/horizon motion enabled), and at "1" the effect is fully enabled (or 100% of roll/horizon motion enabled).  On ovals, some people have reported better success with this value set to a moderate decimal percentage, such as 0.35 (or 35% roll/horizon motion), while on road courses the full value ("1" or 100% roll/horizon motion) seems to work well.

Some examples of this in action:  

Turn 5 at Laguna Seca -- default view (DriverHeadHorizon DISABLED):

Turn 5 at Laguna Seca -- active view (DriverHeadHorizon ENABLED):

Entry to The Corkscrew at Laguna Seca -- default view (DriverHeadHorizon DISABLED):

Entry to The Corkscrew  at Laguna Seca -- active view (DriverHeadHorizon ENABLED):