A Retriever acts like a remote Filter. When activated by a redstone pulse, it searches all inventories attached via Pneumatic Tubes connected to its front face for the items in its inventory and pulls one set from the connected inventories. When the items reach the Retriever, they are pushed out through the back. It needs Blutricity to operate.
Connect the front of the Retriever to a Pneumatic Tube of some sort. Connect those Pneumatic Tubes to at least one inventory (a chest, a furnace, other machines, etc.). Connect the back of the Retriever to an inventory or additional tubes that connect to an inventory that can accept the requested item(s). If the Retriever is not connected to a valid inventory, it will jam and reject all incoming items until the jammed item has a valid destination.
Connect the sides of the Retriever to a) a source of Blutricity and b) redstone or redwire. Right-click on the Retriever and place in it one or more of the item(s) you want to retrieve. The retriever will only request items in the stack size that you specify. If you request a stack of 6 items, the connected inventory must have at least 6 of them or none will be retrieved.
Activate the Retriever with a redstone pulse and the Retriever will retrieve an item from an inventory connected to its front and send it to an inventory connected to its back. If there is not enough Blutricity to process the request, the retriever will not request an item.
Items pulled into a tube network by a retriever will follow normal destination rules and may not end up at the retriever that requested them. Normal destination rules are: Items will go to the closest valid inventory. That means any chest, retriever requesting the same item, Filter, etc.