Pieces on boards in the present are also able to check any available king on every board in the past, present, or future that is within time-jump range of that checking piece. Because kings are able to evade checks through time travel but limited through the 1-space movement = 1 timeline movement rule, you're able to force your opponent by wasting a move on their best timeline by moving their vulnerable king from another timeline INTO that best timeline. That brings them to a weakened position where they have multiple kings within that timeline, which can make checking them significantly easier. As they are still limited by that 1 move = 1 time move rule, forcing an opponent into having 2 kings onto 1 timeline means that it is much easier to check them on that timeline.
They have spent like 70+ minutes on this one game and have cumulatively moved only like 12 moves through a singular timeline. At this point one of them would be punished for constantly weakening timelines by constantly creating new timelines but they both keep making weakened timelines by bolstering new timelines. They refuse to play in the future but keep going to the past. Kronii didn't even realize she had 5 knights on one timeline until Ame told her.