Later you hear the recording and the trombone notes sound separated. "What's going on?" you think. "I know we were sustaining those notes..."
Look at the diagram below:
Everything above the line can be heard out in the hall. Everything below the line may audible where you're sitting but not out there.
If you're not really sustaining the dynamic the effect is separation.
Consider the reverse: if a younger brass section tends to "wah" their attacks...
In this case, they started the note on time but, to the conductor, it sounds like they are coming in late.
"Get right to the ah of tah"