When the Bnei Yissochor zy”a would daven for the amud, he would have an Arizal Siddur open in front of him, but he wouldn't look into it. Instead, he davened from a different siddur.
When his students asked for an explanation, he told them the following story:
The Baal Shem Tov Hakadosh zy”a once looked very upset and agitated when he walked into his bais medrash before Kol Nidrei. After reciting Kol Nidrei, he went to rest for an hour. When he returned to the bais medrash, he looked like a new man. He was full of joy, and he sang the tefillos with a happy tune.
After Yom Kippur, his students asked him about his change of mood, and he said, "In a certain city, there lived a tavernkeeper who was greatly beloved by the local Poritz. When the man and his wife passed away, they left behind a young son. The Poritz felt bad for the orphaned child. He took him into his home and raised him as his own son. Thus, the boy grew up without knowing anything about Yiddishkeit.
"When the boy got older, the Poritz informed him that his parents had been Jews. He handed him all the possessions they had left behind, including a few Chumashim and a Korban Minchah Siddur which his mother had davened from.
"When the days of Selichos arrived, the boy saw the local Jews preparing to set off on a journey. He asked where they were going, and they told him that they were traveling to the big city so that they could pray with a large congregation on the holiest days of the year. The boy was struck with an intense desire to join his fellow Jews and find out more about his heritage.
"That night, he had a dream, in which he saw his ancestors calling out to him and telling him to return to his faith. He woke up the next morning in fear and confusion. He told the Poritz about his dream, but he dismissed it and said, 'Dreams mean nothing. Don't pay any attention to it.'
"However, the dreams persisted. Every night of the Aseres Yemei Teshuva, he had the same dream. On erev Yom Kippur, he again saw the Jews preparing to set out to the big city. When he asked them about it, they explained, 'Tomorrow is the holiest day of the year. It is the day when we are forgiven by G-d for our sins.'
"These words made a tremendous impression on him. He went to his room and took hold of his mother's Korban Minchah Siddur, and ran to a city where he knew there was a shul. He entered as the kehillah was beginning Kol Nidrei, and he was confronted with a sight, the likes of which he had never seen before. All the men were wrapped in white garments, with prayer books in their hands, as they emotionally prayed and wept. The boy also began to cry bitterly. He mostly cried because he was saddened by the fact that he didn't know how to pray.
"This caused a great commotion in Shomayim. The Besht felt the boy's pain as well, and he davened that his teshuva should be accepted and that he should fully return to his people.
"While all this was going on, the boy calmed down, opened his siddur, and said, 'Master of the World, I don't know how to pray. I don't know what words to say. Here is my prayer book. Please take the right words.'
"He put down his head and cried for a long time, and his prayer to do teshuva was accepted."
The Besht said, "When I saw this, I was filled with joy."
The Bnei Yissochor concluded, "I also don't know what to do. I don't know the right kavanos to have in mind. Therefore, I put the Arizal Siddur in front of me and open it up because all the kavanos are in there."