It's just a matter of rule of thumb and basic mental arithmetic, as long as you pass the slot at above 200 m/s when appraoching in a remotely straight line, NPCs won't technically have the time to complete a seven second scan starting at their typical distance from station.
Never did the actual maths and the required speed might be quite a bit lower.
Anyway - it's a really safe estimate, easy to do and running silently unnecessarily increases the risk of hull damage when scraping the slot whilst it's just unnecessary.
They just can't complete a scan in that timeframe (scanning stops once through the slot).
Seems to me then, the key advice (whether one opts to speed into the slot or sneak into the slot) is positioning prior to being within mass lock range of the station. I prefer Silent running, personally. I have no trouble getting my anaconda in without scans or damage. Others may prefer boosting into slot with the added safety of shields on. To me it's the same (risk and skill wise).
There are risks and benefits to both styles that I can think of. The advantage of properly used SR is you won't get targeted at all, not even briefly. Unless something has changed, some missions will fail or deduct from overal pay if a scan even begins. In those instances, SR is clearly the way to go. In others where a full scan is required (just being wanted or normal smuggling) then keeping your shields is best. At CGs, it is certainly the safer method on account of gankers who would just plain ram you to death the second you're spotted. Anywho, I'd say if someone wants to get practiced with avoiding scans on the way in to dock, practicing the lineup from supercruise and then again when lining up the slot will provide the most important skills regardless of preference for boost or silent run.