Answer
In a simultaneous exchange and completion, the two take place on the same day.
The exchange and completion of the contract on the same day you intend to move will likely lead to some confusion because a contract does not become legally binding until it has been exchanged.
As such, if you are exchanging and completing on the same day you will not have a binding contract in place until the day you are due to move in.
Sometimes simultaneous exchange and completion happen when there is a tight deadline to complete the sale/purchase.