in the assignment there was a step asking us to do this
Step 1: Apply mask D2 to shut down the same neurons as during the forward propagation
can i understand how does this work and what does applying mask D2 mean?
Hello @zheng_xiang1
I think you were asking about Course 2 Week 1 Assignment 2 Exercise 4. Next time, please also share this information with us 
As for how to “apply”, you have actually done it twice in Exercise 3, when you did this step → “Step 3: shut down some neurons of A1”
Basically, however you have applied it in exercise 3, you just need to apply it in the same way in exercise 4. In exercise 3, this step forces some elements in A1 and A2 to be zero. In exercise 4, this step forces some elements in dA1 and dA2 to zeros. Note that, whichever elements in A1 was forced to zero, the same elements in dA1 should also be zero. This makes a lot of sense! Why? Because those elements were dropped out and were NOT supposed to learn. How to avoid it to learn? Make sure their gradients were zero by “applying” the mask again. That’s why you need to do it again in Exercise 4.
Cheers,
Raymond
ok fully understand that aspect now and i will remember to be more explicit in stating the origin of question, tks raymond