Boss Leading

From The Danmaku Gameplay Wiki

In PCB, IN, MoF, and SA, you can somewhat control the horizontal movement of bosses. For most patterns with boss movement, when a boss has an opportunity to move, their horizontal movement will be towards you, unless doing so would take them outside of their movement bounds. (In most cases, if they can't move towards you, they'll instead move in the opposite direction. Ran's final spell and Yukari's penultimate spell are notable exceptions to this. More information can be found on their respective pages.) Vertical movement is usually random. The distance that a boss moves horizontally can also be random. If a pattern has bullets spawning based on the boss's position, you can use this to control where the bullets spawn. Certain patterns, such as Marisa's second nonspell in IN, and Eirin's 2nd spell in IN, can be made much more predictable by always leading the boss in the same directions.

In games after SA, most bosses instead have a 2/3 chance to move towards you, and a 1/3 chance to move away from you, making it harder to control them.