Many people believe rabbits find being placed on their back relaxing, but this couldn't be further from the truth. As prey species rabbits have evolved various tactics to help them avoid predation. One of these is called 'tonic immobility' - sometimes referred to as 'trancing' or 'hypnotising'.
Tonic immobility is a rabbit playing dead
If a rabbit is captured by a predator they become still and apparently lifeless - in effect, they're playing dead. By remaining motionless a predator may believe the rabbit is dead, so relaxes their grip and offers the possibility of escape. This state can also be induced when a rabbit is turned either partially, or fully over, onto its back.
Rabbits go into a state of stress when on their backs
Rather than being relaxed, rabbits show both physiological and behavioural indicators of stress and fear prior to, during and after being turned over. Studies have shown that rabbits flatten their ears, widen their eyes and tense their muscles when being placed in this position - which is them expressing worried and uncomfortable behaviour.
Whilst on their back, rabbits remain fully alert to take advantage of any escape opportunity. Once back on their feet, they'll spend more time hiding and less time exploring. They'll also have an increased heart and respiration rate and higher levels of the stress hormone, corticosterone.
Staying still isn't a sign of relaxation
The more often a rabbit is laid on their back the faster they become still. This shouldn't be mistaken for them enjoying or habituating to the process. They've actually learned to anticipate what is about to happen and repeated exposure to this procedure causes them to become stressed quicker.
Keep rabbits upright when grooming
Due to the fear and stress experienced by rabbits when placed on their back, things like general interactions, routine health checking and grooming should all be undertaken without putting them in this position. However, vets which need to carry out certain procedures like taking x-rays or a thorough examination of teeth may utilise this response if it avoids sedation or anaesthesia which can present risks.