The obligation, stated plainly
The prohibition on returning a person to a place where they would face torture is among the few rules in international law that admits no exception. It applies in peace and in emergency, and it applies to every person regardless of conduct or immigration status. It binds the state that transfers a person, and it is not discharged by the fact that another state agreed to receive them.
Two consequences follow that are easy to miss in the moment. The obligation follows the person rather than the paperwork, so an arrangement between governments does not remove it. And it extends to onward movement: a state that transfers a person to a country from which they are then sent home to the danger they fled has not avoided the question, it has answered it.
What careful practice looks like
The safeguards that appear repeatedly in the case law and in the guidance are procedural, and they are the ones that can be evidenced afterwards. Notice to the person of the country to which removal is proposed. A real opportunity to say that they fear that country, before the movement rather than after it. A decision by someone who was not the person proposing the removal. A written record of what was decided and on what material.
Where assurances have been given by a receiving state, the recurring questions in the literature are whether the assurance is specific, whether anyone is able to verify it after arrival, and what happens if it is not honored. An assurance that cannot be checked has been treated by monitoring bodies as worth very little.
Why the record protects the official who keeps it
This is the practical point, and it is not a threat. Decisions taken under pressure are examined later by people who were not there and who did not feel the pressure. What survives that examination is the file.
An official who recorded the notice given, the fear-based claim asked for and answered, the material relied on and the person who decided, is an official who can show what was done and why. An official who has none of that is left with a recollection against a document. Governments change, inquiries follow, and the individual who kept the record is in a different position from the one who did not.
There is a second reason, less often said. Most people who take these jobs do so intending to administer the law rather than to be an instrument of harm. A clear record is what makes it possible to demonstrate that, and it costs almost nothing at the time.
Where the obligation comes from
The material behind all of this, with sources, sits in the project pages. The Coalition would rather an official read the instruments and the decisions than take its summary on trust.