From Pluralpedia, the collaborative plurality dictionary
headpressure (n.)
Applies tosystem functions

Headpressure is a pressure, lack of focus, or dizziness that is common among systems. It has multiple causes, including too many members being co-conscious at the same time, and other members trying to front or cause a switch.

It's sometimes described as being like a headache. Some systems experience this without the pain associated with one, while others still have the pain.