Avoids the full force of a lightsaber blow to the head through superhuman reflexes

Leia appeared to hesitate, then lunged straight at Vader with the tip of the saber. Simultaneously the
Dark Lord brought his own beam up in a lightning gesture to parry hers.
But Leia performed a spinning, twisting arc in the air and brought her saber down in a slashing flare of
blue light. Energy flashed as it contacted the Dark Lord's armored breath mask. Only superhuman
reflexes enabled him to avoid the full effect of the blow.

Splinter of the Mind's Eye, Chapter XII, 120