In politics these are called dog whistles. You speak vaguely enough to allow plausible deniability later on, but you put something out there that people will hear a certain way. Those particularly devoted might hear it as a call to action. If you watch the video replay of Trump's speech, you can see a bearded gentleman in a red polo shirt react to the "Second Amendment people" line. Whatever Trump's handlers say he meant, this man knew the potency of that remark.