Why does Tsuna lose his clothes when infused with the Dying Will? 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?
Anime often ignore real world's Physics laws. The losing of clothes on being infused with the Dying Will is used to dramatize the effect it has on Tsuna. It is to make you feel as if Tsuna's body "grows" when he receives the Dying Will, causing his clothes to rip apart.

Also, in a number of cases, receiving the Dying Will causes Tsuna to move from a lying down position to an upright position. This motion may also cause the clothes to rip, since they cannot "keep up" with the Dying Will.

Another explanation is that receiving the Dying Will is so intense that it tears or burns things in his immediate vicinity (which happen to be his clothes). I think this is a case of Rule of Cool in addition to Rule of Funny, and Fanservice could also be another reason.

The underwear remaining intact is for censorship reasons only. See Magic Pants.