It relates the thrust, weight, drag, wing area, and other flight characteristics of an aircraft into a quantitative model. This allows combat capabilities of various aircraft or prospective design trade-offs to be predicted and compared.
Specific power equals thrust minus drag, divided by the aircraft's weight, times the current velocity. Power is a force times a velocity; any parameter termed "specific" is normalized to either the mass or weight, hence the "s" subscript.
Boyd, a skilled US jet fighter pilot in the Korean war, went on to pioneer the use of computer modeling to predict aircraft performance. His work at Eglin air force base in the early 1960's, charting the performance envelopes of US and Soviet aircraft, produced conclusions that were unwelcome but ultimately undeniable. Boyd's theories, coupled with his persistence, brought about crucial design improvements in the F-15 and F-16 jet fighters.