The concept is very simple: a laminated card is attached to the passenger-side door mirror, a number of orienteering punches are set out around a site and the aim is to visit and stamp your card with as many punches as possible. As the card is attached to the vehicle's mirror, you need to get the card to the punch.
Each punch has a different pattern of pins to mark the card, and the card has numbered boxes corresponding to the numbers on each punch. It is a simple matter then to match them up: e.g. mark box A3 on the card with the punch labelled A3. Simples. The person with the most punches in the shortest time is the winner.
Of course it is the location of the punches plays a big part, and this is where the challenge element comes in. They can be anywhere: up embankments, across streams, in deep water, through narrow gaps in trees, in ditches etc., so the skill of the driver, and the type of vehicle, plays its part too.
A heavily modified 4x4 with larger wheels, a winch, raised air-intake etc. will find it much easier than a completely standard vehicle, so a handicap system is employed. At the start of the challenge, each entrant is told how many punches they need to collect: the more modified the vehicle, the more punches will be required. This allows the unmodified entrants to avoid some of the more extreme punches that they are incapable of getting to.
Whilst it is possible to do it while single-crewed, it is much easier to have a co-driver (AKA "winch-bitch") in the passenger seat to punch the card, look for the punches (which are often hidden), guide you in to tight spaces and, if necessary, help you winch yourself out of trouble.
Depending on the site and how the punches have been laid out, it can take from a couple of hours to most of the day to get them all. Even if you are not especially competitive and are just doing it for fun, the opportunity to get a punch gives a purpose to the day and gives you an incentive to stretch yourself to have a go at things you might not normally consider trying.
The nature of the event means that you will be putting yourself and your vehicle at a degree of risk in terms of damage, but with care and patience it should be possible to emerge unscathed - although most Land Rovers are not watertight, so fording deep water is likely to result in wet feet (don't ask me how I know...!).
Whether you win or not, it's the taking part and having fun that counts!