For me the turn on of the need for a bowel movement is the naughtiness and thrill of putting off going to the boy's room.
I am an habitual nappy wearer - nappy knickers (as many pairs inside one another as I want) and then plastic pants.
I usually use six glycerin suppositories to induce the urge to empty my bowels. I find the delay between insertion and the onset of cramps is not dependant on the number of suppositories. Whether I use one or six it is always about twenty minutes before the laxative effect starts. This means there is plenty of time to pull up my nappies and dress in shorts and a shirt.
Of course I have no intention of going to the boy's room. Deliberately pooing one's pants seems to be a social taboo and as such is frowned on. I find the naughtiness a turn on. I usually become sexually aroused and erect at the anticipation of the onset of cramps.
Then, when the cramps do start I play with them. Clenching my bottom and holding my breath to control the urge. In the lulls between spasms I let my bowels lie heavily on the inside of my anus. This is sure to precipitate further spasms. Stroking myself through my nappy helps.
If I only use one suppository I find the laxative effect wears off after a short while and I am left with an anti climax. (This can be solved with more suppositories or a Fleet.) With six suppositories the laxative effect lasts much longer and almost always gets strong enough to cause loss of control.
When the spasms get very strong and I desperately need relief I find I have to stay very still. If I am alone I might sit and move my pelvis in my nappy so that I masterbate. Playing with my nipples helps.
This is when, in society's eyes, I should be going to the boy,s room. Instead I don't and, of course, more spasms invade my tummy. Then it is too late. I have put off for too long. The spasm that beats me ties my bowels in knots and I fill my nappy.
This is the best part. I can relax as I don't need to fight the laxative any more. I fill my nappy and even when my bowels are empty they are still in spasm. I let the cramps wash over me. I savour the thrill of feeling the seat of my nappy bulge.