[@Prisk] I was thinking in terms of temporarily weakening/strengthening it either in a small area or on himself (for example, treating his own body as having a much higher weight at the point of an impact to make a blow much heavier or to reduce the effect from a fall by reducing his weight and so his terminal velocity).

I realise acting on even a small area would be open to abuse, hence my examples for enacting the effects solely on himself.