Judy asked “What’s a clapper?”
Threads posted a great article titled Five Must-Have Sewing Tools. Point presser/clapper is number five on the list. The article explains how to use it and provides pictures. I wish I’d know about this tool years ago.
This tool call also be called a tailor’s clapper (read the last to paragraphs), combination clapper and point turner (scroll down to find the reference).
Here’s a good picture from sews.com where you can order your own clapper. Nancy’s Notions sales a tailor board & pad that I might get instead.