You should really have both hands on the wheel when reversing. This is especially important when turning the wheel.
Putting an arm behind the passenger seat is not advisable unless - the driver has difficulty turning round whilst keeping both hands on the wheel due to medical reasons or due to their build.
If it is necessary to put the arm over the back of the passenger seat - balance the clutch first, release the handbrake and then put the arm over the back of the passenger seat.