Example

here is a 3rd down defense, there is a star (Ni) and a Money (D)
let's say Lake is Ni and Curl the Money --

It's a classic Fangio scheme with the weak side safety robbing the strong side (TRIX)
he's looking for any routes crossing from the strong to weak side.

Sure, it's zone, but the RCB is in man.
The LCB is supposed to force WR inside, but in the slot goes deep he has in man to man
if they don't cross the he has the #1 receiver man to man and the Nickel has the slot
man to man (it says carry if all-go)

the left safety caps the most dangerous route
the Dime LBer (money) takes the RB in man and if the RB blocks --- the he "looks for work" reads and helps
someone next to him

the MLB, actually could blitz (to keep RB in) or he takes the TE and presents him from crossing (Wall) while the TE is in his zone but "delivers" passes him to the robbing (TRIX) safety ...

So, you tell me --- if the CBs and Ni are really in man coverage --- wouldn't they need man skills? The are just not taking them in the no-cover zone short ...
where WRs cross or mesh ---