1. Usage of when doing
When the subject of the when clause and the subject of the main clause are the same person or the same people, the predicate verb of the clause is in the form of be verb + doing. You can omit the subject of the clause and the verb be, that is, when doing sth means to do... things when, such as:
when doing the work,we should be careful.
A more complete statement is when (we are) doing the work, we should be careful.
When working, we need to be careful.
2. Usage of when followed by do:
(1) When introducing an adverbial clause, it is usually followed by a subordinate clause. When it is in the simple present tense, the do verb form is used. like:
when you do your homework，you should be careful.
When you do your homework, you have to be careful.
(2) When introducing questions, simple present tense
when do you do your homework？
When do you do your homework?
Extended information:
When, while, if, once, unless, and adverbial clauses are introduced, when the subject of the clause and the subject of the main clause are the same, the subject of the clause can be deleted, and the verb adopts the modern participle form doing.
When is followed by doing. The action of doing represents an ongoing action and has an active relationship with the logical subject.
如：When you drive a car，you should be alert.
You can write: When driving a car, you should be alert.
When you drive, be careful.
See https://zhidao.baidu.com/question/1806312054849138747.html