Preching means book distribution.
Preaching means to spread the glories of the Supreme Lord.
The most important aspect of preaching is to take care of the person very nicely.
Simply read my books and repeat what I have written then your preaching will be perfect.
The sankirtana devotees are very very dear to Krishna. Because you are doing the field work of book distribution, Krishna has immediately recognised them as true servants. Just like during war time, a farm boy or ordinary clerk who goes to fight for this country on the front immediately becomes a national hero for his sincere effort. So Krishna immediately recognises a preacher of Krishna consciousness who takes all risks t deliver his message.