This engine uses torque to yield head bolts, they cannot be retorqued as they stay in their plastic state once torqued down. That is why it is critical to use a torque ratchet to get to the first torque of the bolts by following the torquing sequence and then the final tightening passes of the bolts are advised to be done in "ONE" motion only - that means that if the tightening is for 90 degrees, the mechanic needs to set his tool and workplace that the one fluid motion will complete the action, he should not do two 45 actions - that would render the bolt bad.
The above reason is also why toyota actually tell you to measure the bolts and discard any that are stretched beyond specification.
It is also highly critical to use the correct tool for the bolt heads, its not a regular "baar pael ki L key" - which usually means triple square. Toyota use bihex - it is 12 edged but the intersecting angles of the edges are different.
trying to use an triple square on bi hex bolts usually lead to broken bolts or broken tools
click here for my old thread on this