He was a great improvisator and was indeed creating new musical phrases while standing at the "Omud" (altar) and praying .
The late Avrohom Himelstein who was a great composer and the choirmaster of the Great Synagogue in Johanesburg, South Africa and who was also my teacher told me, that he used to go to Shlomo Mandel and induce him to record on a old 78 tours machine while improvising. Shlomo Mandel obliged him. Himelstein later gave me those recordings on tape. They are amazing.
I once asked Shlomo Mandel what was his secret ? He answered me: "Conviction, my friend" - "too many Chazonim are just standing there and singing music" - "I he said, am trying to feel the depth and the meaning of the words I am uttering and just put my Chassidic Neshomoh (Soul) into them and bring them out so that the congregation should feel them as I feel them myself".
Shlomo Mandel was very musical. He could modulate into different moods and always come back to the root of the previous phrases.
Once on a visit to Israel, he was invited by the Israel Broadcasting Company, to sing on the radio. He took a taxi from Tel Aviv to Jerusalem where he was due to broadcast and on the way he composed a piece which made him famous: The "Atoh Chonein".
I was a teenager at that time studying in a Yeshivah in Israel and I used to listen every Wednesday evening to the weekly Chazonus program. When the radio aired the "Atoh Chonein" for the first time, I was immediately taken by this piece. It has a Chassidic leitmotiv which keeps coming back and in between Chazonus which fits to it very naturally.
This piece was so popular, that for months the radio broadcasted it every week.