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.