What's the difference between exploding a subject heading and just selected the top subject heading?
There is a big difference between selecting the top subject heading, and exploding the subject heading. Let's use some screenshots to understand further:
I searched for breast cancer in Ovid Medline. This mapped us to the subject heading Breast Neoplasms, which I clicked on to see where it is situated in the subject heading tree. This brought us to the screenshot you see above where we can see that Breast Neoplasms has many narrower subject headings, from Breast Carcinoma In Situ, to Unilateral Breast Neoplasms. We can also see that Breast Neoplasms is a narrower term of Neoplasms by Site.
If I were to continue with the current selection, we'd be retrieving 351201 articles - all of these are being retrieved because they have been tagged with the subject heading Breast Neoplasms. This search would not retrieve articles that have been tagged with Breast Carcinoma in Situ, Male Breast Neoplasms, or any of the other narrower terms seen in the first screenshot, and not Breast Neoplasms.
If we wanted to expand our search, perhaps because we are interested in ALL types of breast cancers, then exploding would be a suitable option. By clicking on explode, we are capturing articles that have been tagged with Breast Neoplasms AND/OR any of it's narrower subject headings.
Exploding the subject heading Breast Neoplasms is the same as manually selecting all of its narrower subject headings. Clicking on explode saves you that step!
In sum, you will be retrieving more articles if you explode a subject heading (provided that it does have narrower subject headings).
We can see above that Breast Neoplasms on it's own retrieves 351201 articles. Exploding Breast Neoplasms, as we can see in line 2, brings in 365320 articles - way more! Just remember that you want to ensure that the narrower subject headings you are capturing by clicking on "explode" are relevant to your search!