1. When the bad guys light the well of petroleum, Indy tips up the coffin while Elsa is holding the torch.
2. Once the coffin is floating in the petrol both Indy and Elsa dive under in to the coffin.
3. Next scene you see is both of them underneath the coffin with the torch burning behind them.
4. Apart from being very dangerous how exactly did they manage to get the torch under it?