The two previous answers detail two DIFFERENT workout protocols.
Answer # 1 details HIIT, which is High Intensity Interval Training and as answer # 1 suggests the "interval" is necessary for this to be considered HIIT.
To achieve HIIT, train 90 - 100% of your maximum capacity (not your MHR which is really only a guideline and does NOT necessarily measure your maximum capacity) for an "interval" (usually 30 to 60 sec) and then rest for an "interval". Your work to rest ratio really depends on how long you can sustain maximal effort and how quickly you can recover to be able to perform the next work interval at maximum capacity. Common work/rest intervals are 30s/30s, 30s/60s, 60s/60s, 60s/90s.
It is very important to note that the work interval REALLY needs to be maximal or close to maximal effort in order to perform HIIT correctly. Using sprinting and the 30s/60s interval as an example, you would sprint for 30s as if you were running a race i.e. "as fast as you can". Then you would rest for 60s and that is 1 complete interval.
The reason I disassociate MHR and maximum effort is that MHR is really only a guideline. As an example I am a 42 yr old male so accordingly my MHR would be 178 (220 - age). However I normally hit 200 BPM during each work interval of HIIT and recover to about 160 BPM using roughly a 60s/60s interval. As you can see, 60s at 178 BPM is NOT my maximal effort and would therefore not be High-Intensity but would be a medium intenisty interval program or simply known as interval training (before HIIT was invented).
For answer # 2 the writer is really referring to High Intensity Training. If you do not use the "interval" then you are simply doing Hit Intensity Training (HIT) and you are NOT doing (HIIT).
HIIT, HIT and plain old interval training all have benefits but if you want the fat burning benefits of EPOC and the benefits of raising your VO2-Max, then HIIT is the one for you.
Remember that HIIT is really about working in the anaerobic threshold, recovering, and working into that threshold again. If you make the work interval too long then you are really working the upper limits of your aerobic threshold which will not produce the benefits of HIIT (or at least I know of no studies which show this). Most experts agree that any activity lasting longer than about two minutes has a large aerobic metabolic component. You do NOT want an aerobic component in your HIIT workout, you want only an anaerobic component so you must go at maximal effort. Otherwise the workout is not HIIT.
If you want to do HIIT keep your work under 2 minutes per interval, Work at maximal capacity, recover sufficiently to perform the work at maximal capacity and repeat. Use the common intervals listed above.