If you're starting with a full rack of spares, and you want to get to "St Louis," you have to remove the "chine" (if it's been left on), the "tips," and the "flap." Some people also take the "small end" if the bones aren't running parallel or there's a large flap of meat that will dry out over a long cook hanging off the end.
About the flap:
Remove it by bending it back, and starting your cut from the widest end, leaving about 1/2" attached to the slab. Then slather and rub the flap meat, just as you would the slab. Like some of the other guys have said, the flap will cook faster than the rest of the slab. (That's a big part of why you trim it off.) To give you an idea of how fast, I usually cook the slab for about six hours and pull the trimmed flap between the third and fourth hour. It's usually easy to get the silvery-white membrane (the "silverskin") off the enitre slab before trimming the flap, by starting a corner, grabbing it with a paper towel (amazingly grippy on the membrane), and gently pulling it off. Sometimes the membrane won't come easily and tears. If so, take the flap and do your best to peel it off by starting it with a thumbnail or dull knife. If that doesn't work, try a sharp knife (which is how I do it).
Your question about the flap is so basic it begs whether you know how to do the rest of the trim. If not, this is a great place for advice. But, before you get all nuts about doing the full trim a lot of people (me!) think ribs are best with the tips left on. They're awkward but luscious.
Hope this helps,
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Klose Steak Grill with Swing Set
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