In each ear there are three semi circular canals positioned at right angles to each other. They are angular accelerometers that sense changes in velocity in roll, pitch and yaw. The canals are hollow rings of bone filled with a fluid. There is a swing door called the cupula, across the cross section of the bony ring. Relative movement between the fluid and the canal causes the cupula to be deflected. The cupula is connected to a nerve that sends movement signals to the brain.
If the canal is subjected to an acceleration the swing door will be deflected and a message sent out along the nerve. If the turn rate becomes constant then the cupula will return to the centre and a no movement signal will result. Subsequent arresting of movement will mean the inertia of the fluid will deflect the cupula in the opposite direction and cause a sensation of movement in that opposite direction. It is also possible, due to the viscosity of the fluid and friction of the walls, for accelaration to be so slow that it goes undetected. This is called sub-threshold stimulation.
Just about everyone has experienced the leans. It is easily recognised and in most people can be corrected for. It can occur in a number of ways and the result is a false sense of roll attitude. The most common way for it to occur is by allowing one wing to drop at a sub threshold rate, thus the brain still believes the aircraft to be level. The error is noticed and the aircraft quickly rolled back to wings level, this fools the brain into thinking that we have rolled into a turn. There is then a strong conflict between what the eyes see on the AI and what the brain thinks is occuring. The effect can be quite overpowering and people have been known to lean in their seat to try and correct the problem, hence the name. Eventually the brain will come around to realising what attitude it is actually in but until that time attention must be paid to the AI otherwise you will roll back into a turn in the original direction. The only ways to try and combat the problem are minimising head movements, maintain a high proficiency in instrument flying (so you don't get into a UA in the first place!) and transition onto instruments early. Remember experience DOES NOT make you immune.
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