What is: Tenuki

TENUKI: An employee is not doing what he should do (he should be adding value, hataraki). He is doing something else, so wasting his energy (ugoki)

Tenuki could be translated as ‘absence’. Your first association would be with illness. But this is not meant here.
Here the employee is ‘absent’ doing his real job, because he is working (maybe sweating very hard!) on something else. Something that creates no value; a complete new perspective, isn’t it?