I have posted this (procedure) a couple of times:well the main reason is because they can flip themselves in the normal position and hurt themselves
The second, while on the back, they are in trans and have no control over anything which is kind of stressful for them.
Its not recommended to place them on the back by a novice owner.
I have to use kitchen hose (small shower) to wash her privates and there is nobody to help me with that. I have to hold her (on her back) with one arm while another hand is holding the shower. While doing this, I try to get her wet minimally.
1. Plastic garden table, plastic garden chair, small round plastic bowl, container of washing up liquid. Pair of small, rounded scissors. 2 x 5 litre containers with warm water. Some old towels.
2. Have the table to the left of the chair - if you are right-handed.
3. 1/3 fill the bowl with soapy water.
4. Capture your rabbit, sit on the chair and place her on your lap - on an old towel.
5. Lift her up to your chest and move the bowl of water onto your lap.
6. Gently lower the rabbit's REAR END (only) into the water - so that she can have her front legs on the side of the bowl to support her.
7. With your right hand, gently wash/splash water onto her rear. Dip your fingers into the washing up liquid to clean her and continue splashing her rear with the water.
8. To change the water, lift her up to your chest and (with your right hand) empty the water and replace clean water from one of the 5 litre bottles.
9. Continue until she is clean.
10. Lift her up to your chest, remove the bowl and place 1 towel under her and dry her rear with another towel. (Takes 10 mins to dry my rabbit).