With a good freediving suit, the face seal has smooth rubber and seals well, so there is no significant leakage down the neck. Freedive hoods generally seal so well that the diver often needs to peel back the face seal a tiny bit and allow some water in the hood in order to prevent an external ear squeeze associated with air trapped on the outside of the ear drum. This is only needed once at the start of the dive.I read this part right here and my first response was to shake my head and think "No, @W W Meixner's got it backwards, the one piece design including built in hood has less places for water to enter because there's no break between the hood and the suit". Then I read about the inner dam, and thought about that cheap fabric piece that I slip over my head from the front before I slip the hood over from the back and how I feel water rush in the sides of the hood and down my chest and the lightbulb came on. I just might get myself a new semi-dry suit without a hood and get a separate hood.
It is my understanding that most scuba suits don’t seal as well making this step unnecessary.