"Off the top"

I am trying to put together some evaluations of "milestone" services taken by two of our local worship leaders. Although Helen's was a first service and Beverley's was for examination for Accreditation, I noticed something interesting in each:
They both drew on careful and studied knowledge of the biblical text and what commentators have said about it. But they didn't give in to the temptation to give us every last bit of "stuff" they'd discovered: as they related their discoveries to today's situation they preached "off the top of their knowledge".
Both were clearly involved in passionate interaction with their material and their desire to share it with us. But they didn't descend into a style of presentation in which their emotion carried everything before it. We knew they cared but we didn't feel pressured to accept the message because of the emotional content with which it was delivered. They preached "off the top of their emotions".
Our little congregation is blessed to have such worship leaders in its midst.