It just goes 'along' your trousers rather than attempting to go through them. What it's like depends on where it's pointing.
If it starts when your penis is pointing upwards, then all that happens is that it gets restricted to stay that way. IME, it's not unpleasant, and while you're aware you have an erection, depending on the tightness of the trousers and the visibility of your crotch, no-one else might notice.
If it starts when it's pointing downwards, then it's much less comfortable, especially if you're standing. Unless the trousers are extremely loose, people may well notice.