Express Boost uses the same machine-learning algorithms as Express to intelligently calculate the optimal bid per impression, but may place bids higher than the bids you've applied to your targets. With Express Boost, you'll bid competitively to be more eligible for traffic from top-tier channels.
How do I manage my bids in Express Boost campaigns?
The bids you set per target will serve as benchmarks for what you actually bid, so it's important to manually optimize your bids. For example, if a target has a high conversion rate, you should increase its bid to raise ROI. The Express Boost algorithm will marginally raise or lower the bids you've set based on performance data.