Utilizing a take off adapter or "mono cow" allows you to remove a collection flask with out breaking vacuum. This is far superior to the in line designs that simply turn the reeving vessel out of the way. in the case of countervails you are removing the recieving flask of vitals from the geometry of the system and there for remove the possibility of reverse contamination. There are many advantages to utilizing a mono cow. For example you can take samples and easily receive different fractions or parts of fractions with out breaking vac or having to pump power the system.