How do runners conserve ultraboost momentum after checkpointing, like in this video at 0:16

at least before 16d, the way to do this was to checkpoint before the explosion hit you, so that the speed was applied to you after checkpointing (i think).

now its apparently much harder than before, but i have yet to try it​, so i cannot confirm this.

cap your frames to 120 fps, do the nuke and pause, when paused and you hear the explosion, then checkpoint

After the 16d update, the only way you are able to get the checkpoint UB (the carrying of momentum from one point to after clicking checkpoint) is by either frame perfectly timing your pause menu to appear the frame you are first hurt by the Nuke and before the knock-back, or by pause buffering (pausing, un-pausing, and then pausing again) until you are paused on a frame where you are damaged, but not knocked back. While these two methods have achieve the same goal, I personally find the pause buffering much easier to preform, as do others. If you want to be able to preform them easily, as was done in this record, you need to be on a patch of the game before 16d. To figure out how to do this, you can look at this guide, or ask in the discord. Once on this patch you just have to cause a nuke and pause, with the timing being extremely lenient when you set your fps cap to 120. Hope this helped.