I pushed a short sharp blade into my lower abs - nearly 25 years ago. I remember having a hard-on, throbbing as I pushed the blade in - it was at the center of my belly - where the waist band of my underwear would be - and as the blood started to flow - my cock was twitching and spewing cum. It was quite the scene - I do not remember any pain while I did it - and I made a horizontal cut of about an inch (2 cm).
The cut puckered and I could not keep myself closed with just a Band-Aid, so I used a needle and some dental floss to make a few stitches to hold myself shut. A few days later I could use just a Band-Aid. Would like to say it healed without issue, but I did have to lance it - as I started to have an infection at the cut - it took a long time (several months) to heal - and it itched like MAD for a while.
I have since read that the anus will contract while the belly is being cut - would love to share the experience with another - might even cut further - have also read a cut of about 3 inches will cause the bowels to slither out -
Be careful - unless you wish to visit a doctor / hospital / or a mortuary -