This appears to be a term many are familiar with, but how exactly do you cause the beer to foam prior to capping? I use a spring-loaded filler rod, which always results in about an inch of head space, with very little if any bubbles.
If you bottle in PET (ie "plastic") bottles, it's easy, even out of the fermentation vessel. You just squish the bottle until the foam starts overflowing and then tighten the cap. You can also build a cap with a tire stem built into it, and use a hand-held CO2 thing to fill the bottle, but that's a story for another day.
Most of the time capping on foam is a technique used when filling bottles off of kegs. There is always some foaming that happens during the fill off the keg.
Obviously it can't be done if you are using flat beer with priming sugar.
But if you are using some sort of filling method with carbed-up beer that results in not foam you can induce the foam with a quick and sharp tap on the neck of the bottle with a metal spoon. It has worked for me in the past. Its similar to "knocking" the opening of your buddies bottle of beer with the bottom of your bottle of beer. The beer foams all over the place and you have to pound it down. Its a dumb college thing we used to do to each other if someone wasn't paying attention to the conversation.
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