Here is the original item:
While Dr. Kinser bounced down the
stairs (A) and
strode through the hallway.
(B) A brightly colored
tie streamed over his shoulder,
(C) flapping
in his wake.
You wanted to do this:
While Dr. Kinser bounced down the stairs and strode through the hallway.
A brightly colored tie streamed over his shoulder
flapping in his wake.
Removing the comma after shoulder is an error. Flapping in his wake, a participle phrase, needs the comma to connect it to the main clause in front.
You might want to review the rules.