ANSWER:

Sent in by Rajkumaar Perumal of Chennai, India, the answer to this week’s question can be found by focusing on the internal design of an impulse pattern. A standard (trending) impulsion’s internal structure is :5 :3 :5 :3 :5, whereas a terminal impulse is a :3 :3 :3 :3 :3 affair. 

Since a Diametric formation always compacts to a :3 (ie, a correction), the ONLY time a Diametric can occur as the 5th wave of an impulsion is when that impulsion is Terminal in design.