Reach Checks on Anticheats

Question:How do those three block reach checks manage to flag players who are using reach? I've heard that some anticheats, namely Anti-Gaming Chair and Verus Anticheat, can detect players who are using three block reach, or reach close to that setting. That seems incredibly difficult, especially when you consider lag is a thing and frequently screws over anticheat developers.

Does it do something with the packets sent by the client? I have no idea as to how it detects reach.

I think you're right it has to be something with the packets but that also means laggier players will be flagged more so if thats true then i think were right.