This is one of those effects that is meant for a very specific audience and occasion.
It is NOT SUITABLE for children, please!
Ask for a male volunteer from your audience.
Two handkerchiefs are tied together, the knot is tucked (discreetly and tastefully) into the front of the volunteer's clothing, and you and your volunteer each hold the tip of a handkerchief.
You tell your audience that, on the count of three, you and your volunteer are going to pull on the handkerchiefs, making them pass right through the volunteer's body without harming him a bit.
But when you each pull on a handkerchief, a set of BOXER SHORTS appears, tied between the handkerchiefs!
It's a silly surprise that will make everyone laugh, including your volunteer!
This trick is also funny if you use it on yourself.
For example, you could pretend to have trouble handling the tied silks plus other items, and so you tuck the silks into your belt to get them out of the way while you do something else.
When you take the silks out of your belt, the boxers will be seen tied between them.
This trick is also called 20th Century Briefs.