I understand that unlike the Sea Marshall which is just a basic PLB (local reception), modern aircrew PLB's all transmit direct to satellite with a GPS position. The COSPAS - SARSAT system will receive a PLB and triangulate the position within a short period of time - Before that, it should have a GPS position from the device - Within a few mins, Search & Rescue all over the world will be alerted and your Ops Centre will be getting a call to know that you are potentially in trouble.
Unlike local 121.5 / AIS PLB's, these sorts of beacons are not allowed to have automatic transmission. They have to be manually activated - by law.