I think automotive dynamics are of limited relevance in terms of over/understeer in MX.
The most common situation where I have understeer is entering flat turns while still decelerating. It feels like your front tire is sliding out from under you, and is not very nice.
The most common case of oversteer occurs when I gas it out of a corner. The rear steps out. This is often pretty cool, but usually not fast.
Body positioning greatly affects over/understeer, too little weight in the front increasing understeer and vice versa.
Braking and throttle use are more complicated as to their effects. When you apply throttle, you transfer weight to the back as the rear suspension compresses, thus increasing the chance of understeer. However, you are also now using some of the rear tires tractive capacity to accelerate, leaving less for lateral forces, inducing oversteer. You can deduce how braking at both ends affects these.