Google is changing the promotion process such that managers have much more power (for all promotions up to level 5 == senior swe).

Previously it used to be, as Michael says, very much in the hands of the committee members who never heard about you or even your team.

Now the committee will be composed of your manager and some other two managers from a related team. The committee will not see your own promotion rationale or your own description of your projects and achievements, rather it will be the job of your manager to present these points and advocate on your behalf. The committee (except for your manager) will also see only limited peer feedback compared to before -- no free form text, only multiple choice questions/answers.

If all three managers agree that you meet the bar, you get promoted.