For " wheel on" trainers you want to position the trainer tight enough against the tire that it won't slip when you step strongly on the pedals from a dead stop. If it is slipping this can not only give you wonky data, but cause some knee discomfort as well.
They are a specialty training cranks called Power Cranks. Each crank has a clutch in it that makes it so pedaling with one side does not push the other. REALLY works the hip flexors!
