The .explain() method in the shell works with some write methods (update, remove, findAndModify, as of SERVER-14101), but it doesn't work with the newer CRUD API (added to the shell as of SERVER-17953).

I think the missing explain methods are:

> Object.keys(Object.getPrototypeOf(db.c)).sort().filter(x => x.match(/One|Many/))
[
	"deleteMany",
	"deleteOne",
	"findOne",
	"findOneAndDelete",
	"findOneAndReplace",
	"findOneAndUpdate",
	"insertMany",
	"insertOne",
	"replaceOne",
	"updateMany",
	"updateOne"
]

I think this would be especially helpful for new hires: explain is a good way to quickly find the code that implements a command.