Since I was about 10 years old I've masturbated without taking my underwear off and am amazed it's not a technique more frequently discussed. I guess it started when I first began feeling down there and rubbing myself through my PJ trousers or briefs - whatever I had on at the time I guess, but since I started actually masturbating I've always kept my briefs on as it feels loads better than doing it the 'traditional' way.
As for the technique, basically, if I feel horny I will leave my briefs on and allow myself to get hard while my penis is still pointing down to my feet - allowing it to push against my underwear with the briefs holding me in that position. I then put my hand over my underwear and start sort of tapping and rubbing myself which slides my foreskin around. After a while when I cum it feels amazing as I feel my briefs become wet under my fingers. This is a great technique for edging with too as a game I often play is seeing how close I can get without my underwear getting wet!
It'd be great to hear if anyone else ever does it this way - I spent many years thinking I should start doing it 'normally' until I learned it's ok to do whatever feels good!