Currently there are two ways to issue explain from aggregate command:

db.runCommand({ explain: aggCmd });
db.runCommand({ …aggCmd, explain: true });

The first way is the preferred way, while the second one pre-dates the existence of the explain command.

The first way is also an alias over the second way under the hood (as explain command calls parseForExplain() which in itself attaches the verbosity from the explain to the aggregation object)

Explain attribute is not part of the stable API (code, documentation) and can be removed.

PS: Noticed that aggregate::explain type is not boolean (based on idl definition), but rather explain verbosity and this verbosity is used later for explain