Most managers are pretty bad at measuring actual job performance and, instead, default to basing their evaluations on “face time” or “chair time.” This, of course, is silly, and easily gamed. The opportunists work slow and stay late, while the best employees work hard and go home.
If I were to ask to work from home two days a week, I would be sure to present my supervisor with documents and plans so we can mutually set short and long-term performance goals, identifying how and when progress against goals will be measured. I’d also volunteer time diaries and weekly progress reports, and agree that if the boss feels my performance, in any way, declines they can cut back on the flexibility.
In this way, we give our bosses the tools to actually do a far better job in evaluating all employees going forward.