Comma after introductory phrase followed by a verb

While researching the use of commas after introductory prepositional phrases, I came across a document from a university in Texas . In the section for prepositional phrases, it stated, "Never place a comma after a phrase that is immediately followed by a verb."

The given example was

"Into the raging river plummeted the raft with its frightened occupants."

Is this a commonly followed guideline? I've never heard it before. Also, is this guideline the case for any introductory phrase or just prepositional phrase?

For example, if I understand the university's guideline correctly, I should not use a comma in the following sentence because the introductory phrase is followed by a verb. (As a side note, my guideline at work is to place a comma after introductory prepositional phrases of four or more words.)

"With the blue feather write each spelling word in the air." (This example is a spelling activity for children.)

But if it were worded this way, it would take a comma:

"With the blue feather, please write each spelling word in the air."

Or, worded as a statement rather than an imperative--in case that matters:

"With the blue feather, the girl will write each spelling word in the air."

Would I use a comma after this introductory phrase, though, which I believe is a participial phrase? Or does the same guideline apply to all introductory phrase, and I should leave the comma out because this phrase is directly followed by a verb?

"Using chalk, write each word on the chalkboard."

Versus wording it this way which does not have a verb following the introductory phrase:

"Using chalk, Harry wrote each word on the chalkboard."

TIA!