Ive always covered the exhausts of trucks and machinery when transporting them . I'm not sure if it is a downdraft or a vacuum that causes the turbine to spin , but it does happen . I also cover or turn around the ram type air intake snorkles as i'm sure they could cause it to happen . I think it would mainly apply to ball bearing turbo's that spin very easily which would cause bearing damage and maybe not so much on sintered bronze type bearings as the shaft would have resistance to turning without the film of oil but i take no chances and cover them all .