[PD] How does vline~ work under the hood?

[i go bananas]

I want to recreate the timing accuracy of vline~ in a c++ project, and
there's something that i can't figure out exactly - how does it act,
seemingly outside of the block construct, to get its accuracy?

in the source code, there is a calculation for elapsed logical time, and i
assume that has something to do with it?

if i make a vline~ from 0 to 1, over 10ms, does this mean that it actually
doesn't START right on 0, but rather starts at a slightly offset value to
compensate for the block boundary?

any help would be appreciated!
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