The ideal is to wait for the other party to start making concessions, although it is also possible to take the initiative, which grants a certain moral force since one shows a good predisposition to reach an agreement.
The concessions are made looking for an immediate objective, at the moment in which they can have the greatest impact and not randomly. For example, to overcome a blockade, as a sign of good predisposition, or when it is expected to obtain a consideration from the other party. Whenever one makes a concession, it is advisable to wait for the other party to respond in the same way (it is their turn) and not continue making concessions without obtaining compensation.
Concessions should never be made in response to pressure or threat in the hope of calming the other party down. Normally this will not happen, and more often than not the other party grows up at the success of their strategy and continues to press.
It is interesting to keep some room for maneuver to be able to make a final concession when the agreement is about to be closed. It is a sign of good will and also allows the other party to further convince himself that he has achieved a good result.
Finally, point out that we must try to overcome these blocking situations, that one cannot give up at the first difficulty. However, if satisfactory solutions are not found in the end, it is better to break off the negotiations than to reach a bad agreement.