Obviousy this is a pretty decent talent, +10 to all medicae tests, heals 2 dmg on heavy or Criticaly wounded, + 20 to toughness tests to prevent limb loss. My Question is:

When do we take a toughness test to prevent Limb loss? It is my experience limb loss is pretty much automatic.

After looking at the tables, yes, you are indeed right, these tests are usually automatic.

I have no good RAW explanation for how this should actually work.

Personally, I would allow for a successful medicae test by the master chirugen to allow for a second test (at +20), to possibly "not have actually lost" the limb, but only on the crit hits that allow a test. Usually these describe the limb as maimed or effectively worthless. In the case of a clean chop off, not sure how I would handle it.

In case of a clean chopped off limb, reimplanting it might be possible. If the limb explodes... not XD

Thats true, i suppose it could be used to re attach a seperated limb. But when I lost my arm I obviously couldnt reattach it to myself in the field. In most cases it would take medical facilities to reattach it, and they NORMALLY have an apothecary there.

It could be a hangover from DH and RT, where certain critical effects did force a toughness test to see if you kept a limb.