It is actually pretty easy. All user selectable derates are limits during take off, but can simply be overridden by advancing the thrust levers manually. After the take off phase those user selectable derates do not apply anymore and pose no limit or problem during subsequent approaches and go-arounds. Full rated thrust is available at all times.
Why is the derate a limit? With the user selectable engine derates comes a complete set of new performance data, including Vmcg and Vmca. However during a minimum V1 take off with a derate lower than the maximum thrust level (derate 1/2) an increase of thrust on one engine during an OEI case could lead to loss of control. Very rapidly and not recoverable if not expected. At the same time lower V1s might allow one to take bigger loads out of contaminated runways, so there is a plus right there especially during winter operation.
Since derates are considered fixed thrust settings for the sake of performance calculation one can further reduce a fixed derate by combining it with an assumed temperature. To find the optimal combination of derate, ATM, flap setting and speed schedule one usually uses a computer as the possible solutions can be quite a few and ground time is limited.