There was little change in the formal powers of the HRE Emperor. What changed was that many Imperial states were empowered such that some of the Emperor's powers became irrelevant.

Basically, all parties to Westphalia gained exclusive sovereignty of over its lands, people, and agents abroad, and responsibility for the warlike acts of any of its citizens or agents. Politically, each state can now control their alliances, be it among themselves or with foreign powers, as long as it is not directly against the Emperor or the Empire.

To be more precise, the HRE Emperor retained the authority of deciding on taxation, and in theory the questions of war and peace, all in concurrence with the Diet. He retained legislative powers and indirect judicial powers.

Sources: Bayard Taylor's "The Holy Roman Empire", this great website, critical reading of Westphalia, and the treaty itself.

Could you be more specific?