— Official game quote
They are elite soldiers who mostly act as Hitler's personal body guards, elements of their unit also act as recon troops. Trained to a point of almost-perfect similarity to your Spec Ops. But they have recon equipment and can make Cam Nets unusable for 10 seconds. Then they can get the full info on your squad, relay it to their HQ, and then chaos will break loose. Multiple Leibs extend the cam-net down time by adding 10 seconds onto the time until they become effective again.
Aside from that, they are accurate and carry an STG-44, an early assault rifle, and can use its firepower well. They fire in bursts, hurting a soldier multiple times.
They can even acquire priority targets from your ranks! So it makes these guys annoying in the extreme. Take care of them quick or else.
There's a 0.2% chance per action that these guys will instead call direct support on you. You will hear radio chatter and see the Leib holding out a radio, then the text "A German Recon Unit called support on your position!" will appear. After that, paratroopers, arty, a bomb, CAS, or even a V1 will hit your position. If you are lucky, you will just get a smoke screen.