In higher class establishments, this hole will be fabricated with duck tape to prevent genital chaffing.
99% of the time the person on the other side of the glory hole is a man, so be wary.
Thers are rules to follow for using these holes. If you are looking to have someone suck your penis, then you should stand in front of the hole and wave your erect penis around in front of it. If the other guy wants to suck it, he will stick his finger into the hole and tap the bottom of the hole a few times. That is your signal to stick your penis in the hole.
For guys looking to suck penis, they should be on their knees tapping their finger in the hole. You could also open your mouth and stick out your tongue in front of the hole as an obvious signal to the guys who don't know about the tapping.
Most guys who like sucking penises at glory holes also like the cum. So feel free to ejaculate in his mouth. If he doesn't want the sperm, then he really shouldn't be sucking penis at a glory hole. Or he should be looking for the signs that the penis he is sucking is about to ejaculate, so he can pull off it in time.