It's auto triggered when it's full.
Some basic tips:
It's better to exit an level and get the rewards versus dying and getting nothing. The first level with Succubus will probably shred you when you get to it.
I feel like the increased pregnancy chance items aren't worth it since you can just keep having sex to the point it feels more like a chore which is kinda bad in an H-Game.
Having sex with a girl will fill her BP meter.
Filling a girl's BP is generally more important than yours since it does a lot for their damage and often adds a huge aoe to their attacks.
You can't buy anything if you impregnate the shopkeepers so stock up before hand.
Small potions aren't worth buying because they heal so little.
Succubus that are raping you can be attacked and killed by fairies or your party members.
In general it's better to stay back in the late game and let the girls / fairies do most of the work.
Just use Cheat Engine since it takes too long to do anything because the game isn't balanced well.
Balance is this game's biggest issue since at the endgame you're basically worthless since you're so easy to stun lock meaning that you have to just spam fairies and hope that the girls can kill the enemies. The problem with that is that you need to impregnate the girls for more fairies which means that they don't get exp which means that you have to go back and grind their levels. New girls that get added start at level one for some reason and have to be level grinded up too which makes no sense and you're basically paying a lot for extra fairies a day.
In short this is a very grindy game and you will die a lot and repeat the same stages a lot too.