but what explains him surviving a tyrant stab through the chest?
If adapted perfectly, the virus completely recombines the host's cells, strengthening the body and enhancing muscle power and neural reflexes, granting super strength, speed, agility and other biological and genetic improvements such as an enhanced metabolism and higher-density mitochondria.
The virus exhibits an astounding regeneration capability, and virtually no conventional weapon (including explosives and rockets) can damage the host; however, the virus presents no defense against fire, although burns, like all other wounds, heal quickly with no scarring due to the complete regeneration of tissue. In other cases, the virus simply revives the subject with no physical or biological changes thereafter. The virus is mostly lethal to those not adaptable, and will kill the subject sometime following injection with no chance of revival.
http://projectumbrella.net/articles/Prototype-Virus
Wesker knew there was a high chance that the virus would kill him. However, he still decided to take the risk.
He knew the virus would need about 5 minutes to adapt to him, and he wanted Chris and Jill to watch him die, so they would report his death. So he tried to match the tyrant impaling him with the time the virus would need to adapt. Two things could happen:
1) The virus would not adapt, so the Tyrant would kill him before the virus could;
2) The virus would adapt seconds after the Tyrant impaled him, making him heal instantaneously, leaving him practically intact.
Either that or he only let the tyrant impale him once he felt the virus had already adapted to him.
No matter what was in his mind, he was very lucky to see the light of day once again.
That image is a move Wesker has in RE4 Mercenaries, and it's similar to the move Jake has in RE6.