Unfortunately, I wasn’t able to find a score of the cadenza. Its’ length is around three minutes. As already given with the title, Brahms wrote the cadenza for one of Mozart’s concertos.
Suitable to the minor-key, the whole cadenza has a dark and sad sounding character. Throughout most of it, one can hear one hand hectically and fast accompanying the melody played by the other hand. At the beginning, Brahms started of on a very low pitch, slowly working his way up. A very short sequence follows, where just one hand is playing, having reduced the tempo, before the second hand starts playing a fast under-voice again. The theme that follows is a bit softer than the previous ones, it seems to be working in waves, by constantly changing between loud and quiet as well as the amount of keys being played at the same time. The pace gets slower once more and the key changes to major for a very short time. Rather sudden, the tempo gets faster again, a scale is being played down and up, ending the cadenza with a thrill on the highest point.
I personally wasn’t too excited about this cadenza. There was too little variety in the themes Brahms used and even though it lasted for only three minutes, I found it had become rather boring after only a short while. Besides that, with all the other cadenzas I have listened to, I always had the feeling that they sounded improvised, even if they had been written down, and put alongside with the concerto, whereas this one, I found sounded like a small piano piece, which could have been played on its own as well.