Penetration where the thing that is penetrating (usually a penis) is in turn being penetrated (through the urethra if it's a penis) while inside. To avoid confusion, these will be referred to as the socket, being penetrated by the barrel, which is in turn being penetrated by the pin, using barrel connectors as a metaphor.

Usual scenarios

This can happen in a number of ways, so here's a list of common cases and how they look in practice.

Case #1: "The layered finger trap"

post #3975401 post #2437647

Not to be confused with recursive penetration, this one starts with the pin and barrel already mated with each other, before the socket closes around it.

Case #1b: "The deep-milking penis pump"

post #3153618 post #4638943 post #5175612

The usual scenario for a fellatio with urethral tonguing, among other things, where the pin is also inserted into the barrel first, with the socket closing around it afterwards, but both the pin and socket belong to the same person.

Case #2: "The table-turning surprise"

post #3790587 post #3747412 post #2514605

The most common one for vore, including unbirth_while_penetrated, but also a common infestation setup, and potentially usable for slit sex with gender confusion. This one starts with the barrel going into the socket, usually not expecting to be penetrated by a pin while inside.

Case #2b: "The greatest heist"

post #2294630 post #2512088

Similar to the above, but where the pin is actually removed from the socket in the process. This most frequent with infestation, but can also occur with vore as part of prey transfer, and possibly with something else like an egg.

Note the second example above, post #2512088, which uses partial vore to make the barrel the vagina of a second pred. While almost always the case, the barrel doesn't need to be a penis (or penis-like organ like an ovipositor or pseudo-penis).

Case #3: "Concentric mutual penetration"

post #1155768 post #4376248

A rarer situation where penetratration interlock is more or less exploited to allow both partner to fully penetrate one another to the hilt in a symmetrical manner. Not to be confused with the likes of slit sounding/sheath sounding, which are both cases of recursive penetration.

Case #4: "Secure cum channel"

post #2038080 post #5665995 post #4605817

Some form of meeting in the middle, which ends up with both penetration interlock (from the barrel's perspective) and recursive penetration (from the pin's perspective). Can border on thread transfer if the pin is doing most of the distance, like in post #5665995, although both probably shouldn't be tagged together.

Case #4b: "Full-duplex penetration"

post #4694436 post #4014148

Similar to the above, but one side is doing a full all the way through penetration, and being penetrated the other way as it emerges. This counts as penetration tunneling, but even though it's technically interlock from the barrel's perspective, is less commonly tagged as such.

Visual Guide