PHP Code:
new Float:aang[3], Float:vvel[3], Float:pvec[3];
GetClientAbsAngles(client, aang);
GetEntPropVector(client, Prop_Data, "m_vecVelocity", vvel);
GetAngleVectors(aang, pvec, NULL_VECTOR, NULL_VECTOR);
vvel[0] += pvec[0] * 1200.0;
vvel[1] += pvec[1] * 1200.0;
vvel[2] += pvec[2] * 800.0;
TeleportEntity(client, NULL_VECTOR, NULL_VECTOR, vvel);
That should get you going.