English

Use a comma to show a pause after a introductoty clause or phrase.

When a clause or phrase starts a sentence, it often requires a reader to pause before moving on.

Running at top speed Gene chased down the school bus.

Running at top spend is a gerund phrase that is starting the sentence and where a pause is required; therefore, you would put a comma at the end of the phrase where there would be a pause in the sentence.

Running at top speed, Gene chased down the school bus.