High pressure fuel rail connection

I am having trouble with the high pressure fuel rail electrical contact and will have to replace it at some point. I have temporarily fixed it for now. I work in a scrapyard so was waiting for a TD4 to come in so I can recycle some parts.
A 3 series BMW was in today so I had a look at the connection and it looks the same however it had this (presumably) aftermarket addition in the loom. The HP connection runs into it then it goes into the HP sensor.
Hope that makes sense.
Can anyone tell me what it does and what it is? I am not necessarily planning on using it but am very curious.

It's an aftermarket power booster box of tricks. They do boost power, but also can cause the HP fuel pump to fail.

I thought there would be something dodgy about it. Although a boost in power is tempting i would much rather Roary keeps on chugging along quite merrily like he is at the moment. Probably gnna need to change the plug that clicks onto the HP sensor soon, got it ‘temporarily’ fixed with a zip tie around it then another through that and round the manifold. Works great for a bodge.
Hey Nodge, can you tell me if the front doors on the 5 door TD4 are the same size as the 3 door? Thanks

3 door front doors are much longer. In my experience they are a pain to get out of in car parks !!

I have some new 5 door doors or can source 2nd hand ones. What do you need ??

I was contemplating replacing the drivers door on my 3 door as it’s dinged