Fixed keeps your bids exactly where you set them. Amazon won't adjust them up or down based on the likelihood of conversion.
Dynamic Down Only lets Amazon reduce your bids in real time when it thinks a click is less likely to convert. Your bids never go above what's set, but they can go lower.
Dynamic Up and Down lets Amazon both increase (up to 100%, effectively doubling your bid) and decrease your bids based on predicted conversion likelihood.
The AdLabs RPC bidding method, combined with Placement adjustments, is already designed to push CPCs as high as they can go while staying within your Target ACOS.
If you layer Dynamic Up and Down on top of that, Amazon can double your bids beyond what the optimizer calculated, which would likely cause you to overshoot your ACOS target.
