Keeping Pull-ups/Diapers in Place

I have found a solution that works (most of the time) when it comes to keeping a pull-up or diaper on my Autistic three-year-old.  If I put on a pair of underwear over his pull-up, he is less likely to pull it off repeatedly.  I don't know if it works because it makes the pull-up/diaper less accessible or because of the added pressure on his body-I just know it works on most days.

If you have a helpful hint to share, please let us know!