I saw the seconds later aftermath of a lad who decided to put his head in a Harrier Jet, nose undercarriage bay, with 3000psi hydraulic pressure behind the doors. He knocked the microswitch with his shoulder which told the aircraft system to close the doors. The top of his head was just about clean off before the supervisor saw him and pulled the ground release handle . This then blew open the doors, which makes a fair bang!! This was when I looked over and saw him fall to the floor. I had to hold the top of his head on until the emergency services arrived. About 40 minutes. He survived luckily .