An inch into an ass pussy is a second sphincter. This muscle has most of the nerve endings attached to it and will give the most pleasure when stimulated. Some like the short strokes or the rapid, deep strokes. As long as you keep this area stimulated, your partner will love it and invite you back for more.
That looks like a lot more than just the tip of his cock in her asshole. Just as I've told a lot of women when they agreed to let me fuck their asshole that "I am only going to put the tip in". YEAH RIGHT!!!!