In Katekyo Hitman Reborn, whenever Tsuna enters "Dying Will Mode", his clothes tear in half (though he seems to have no shortage of replacements), with the exception of his underpants. This is briefly mentioned on the Reborn Wiki:
The Dying Will Bullet also has the unusual side-effect of destroying all of the person's clothing except their undergarments.
However, I'm not far into the series and really have no idea why this happens (it bugs me every episode). His muscles don't really expand or anything, the clothes just seem to rip and fly off.
Can anyone shed some light on this, or is this a case of "Rule of Funny" as a running gag?