I think he was asking why HK decided to put a bolt hold-open on the 10/40-series of MP5.
This was mandated by the FBI when they requested HK build them a 10mm MP5 submachine gun. Since all HK roller-locked weapons do not have this feature it had to be custom built. Many people say it's a good thing, since it helps to speed reloading when you empty the magazine. But HK purists claim it's unnecessary since it's considered "bad form" to empty a full mag out of an MP5 without doing a tactical reload. Personally, I say use as many rounds as you've got!
This was mandated by the FBI when they requested HK build them a 10mm MP5 submachine gun. Since all HK roller-locked weapons do not have this feature it had to be custom built. Many people say it's a good thing, since it helps to speed reloading when you empty the magazine. But HK purists claim it's unnecessary since it's considered "bad form" to empty a full mag out of an MP5 without doing a tactical reload. Personally, I say use as many rounds as you've got!