Drowning in detail was one of the problems of the Nuremberg IMTs. One of the persistent German complaints about the Tribunals, both in private correspondences as well as public opinion surveys conducted in the American zones was that the tribunals took too long and excessively documented Nazi crimes. This made the proceedings rather dull and hardly the fodder for courtroom drama.

There was though one testimony that managed to cut through some of the apathy in middle of the tribunal: the testimony of Auschwitz commandant Rudolf Höss (often anglicized as Hoess in IMT documents). Höss's testimony and cross-examination recounted in extreme detail and detachment his duties in gassing at Auschwitz. He cited a figure of three million total victims at Auschwitz. This was an incorrect figure, /u/commiespaceinvader has a good explanation in their answer here, but the sheer volume of dead was staggering. Equally staggering was Höss's nonchalance over his activities at Auschwitz, both in his testimony and sworn affidavit.

What made this event more bizarre for the IMT was that Höss's testimony was not part of the prosecution. He was a witness for the defense, specifically RSHA chief Ernst Kaltenbrunner. The SS chief's defense team had hoped that Höss would affirm that Kaltenbrunner had never visited Auschwitz nor had he signed any execution order. Such a defense would shift the blame for the Holocaust from the RSHA to the SS agencies that ran the camp systems, namely the WVHA. This was a stupid blunder on the part of the defense. For one thing, Höss testified that Kaltenbrunner's lieutenant Heinrich Müller signed orders, but that the camps were under the purview of the RSHA and it worked in conjunction with the RSHA. The defense also underestimated Höss's own self-stylings as a loyal SS soldier which worked to make the prosecution's case that the mass murders of Jews and others had a hierarchy and were not the result of rogue officers disobeying or exceeding orders.

A further irony here is that the prosecution was chomping at the bit to get Höss and his affidavit into the tribunal. Prosecutor Whitney Harris had managed to receive the Höss affidavit, but only after the prosecution had rested its case. This meant that the prosecution could not enter it into evidence under the IMT rules. Kaltenbrunner's team calling Höss to testify was an amazing turn of luck for the prosecution. As Telford Taylor recalled in his memoir Anatomy of the Nuremberg Trials:

It is beyond understanding that Hoess was put on the witness stand not, like Ohlendorf or Bach-Zelewski, by prosecution counsel but by the defense. Considering the broad range of Kaltenbrunner's activities, there was little to be gained by attempting to prove that he was never at Auschwitz and never personally signed a death order there. The consequences were not limited to Kaltenbrunner. The awful scale of Nazi terror- produced by a Fuehrer to whom the defendants had pledged and given their allegiance, and by Himmler, Heydrich, Pohl, Mueller, and other leaders of the Nazi government- cast a pall of shame over the defendants. No wonder {Defense Counsel Viktor} von der Lippe described the effect of the Hoess testimony as "crushing" (niederschmetternd).

There were other witness bombshells that managed to cut through some of the dull procedural questions and mass of evidence. Otto Ohlendorf's testimony for the prosecution was shocking for its graphic description of the Einsatzgruppens' activities. But the Höss testimony and affidavit were notable for two reasons. One, it helped to cement the notion of desk-flying murderers among the public. It was hard to disentangle the operations of mass murder from those men like Kaltenbrunner who ordered them after the Höss testimony laid out how these connections functioned. Secondly, the fact that Höss's presence at the IMT was the result of defense counsel undercuts many notions that the IMT was a kangaroo court or some form of victors' justice. There were rules and procedures and the prosecution did not try to change the established rules to get Höss into the record. No one put a gun to Kaltenbrunner's defense team to introduce Höss. The defendants' legal team were free to operate and here, they royally screwed up.