Testing ABV during a run

[butt wheat]

O-Tay! Howdy yall, great site. A buddy plans on doing a stripping run here in the near future. He read that you do a strip run down to 20% ABV, but where do you measure it from? Meaning, do you just collect all the distillant in one container during the run and float the alcometer in that single collection vessel and when it reads 20% its good? Or are they using a parrot?
This is the one thing he is so confused about...Like when they say during a spirit run, they collect 85-60% hearts, where are they getting the reading from? Its hard for him to think they are floating a alcometer in a 200ml jar...
Any clarification would be much helpful...

BW