I feel the need to explain that not all sailors talk the same sailing terminology.
Im one of those sailors, im sure one of my sailing mentors did this to me. If you’ve heard me mention a velosity knock, well others may call it a “velosity header”
A header is when the apparant wind forces you to “fall off” or point further leward in order to not luff your sails so you can keep sailing fast. We do this all the time when we see our sails luffing, we bear away from the wind to refill the sails.
A Velosity Header is when your sails luff, but the wind direction does not change, the wind speed drops, and due to your boats velosity, or speed combined with momentum, until your boat slows the sails will luff, mimicking a header or direction change.
If you do your usual bear away trying to refill your sail, instead you stall the sails.
If you have a wind indicator on your mast it can show the difference between the two. What I do as soon as I identify a velosity header is get back on my course before the luff occoured, and bear off about 5 degrees and try to coast thru the light wind or “hole”
Here is a link to a good article on the subject, from the Lapworth 36 (big boat) website.
Our pond has lots of changing wind conditions, big wind shifts, and constant wind speed changes. It’s those wind speed changes that cause “Velosity Knocks”