It's a pretty simple style of workout, designed to lift your power at threshold as well as provide a bit of a neuromuscular twist. Indeed any solid tempo effort at this level is very good for helping to lift one's threshold power. They are not easy workouts and can be quite fatiguing, so it does help to build up to doing them if you haven't been riding much lately.
There have a been a few different formats but the basic theme is the same:
Ride at a base load that is sub-threshold (~90% of your 1-hr maximal power) and every few minutes do a surge for about 15-seconds or so. Doing this mixes up the workout and does help to make the time pass a little more quickly.
Since today I was planning on a 90-minute solid tempo ride, and it was raining outside, I decided to do my workout indoors on Thunderbird 7 and thought, you know I've never actually had a go at the HOP. So that was today's mission.
Actually I ended up doing what you might call HOP+, since I did want to do 90-min of tempo, rather than the hour. So I decided to do a HOP, then assuming all was well would finish off the workout with some more tempo. This is what eventuated:
This is the picture of the workout data (click on it to see a larger version). The two horizontal dashed lines mark my Funtional Threshold Power (FTP) and 90% of my FTP. Plotted are my power output (yellow) and cadence (green). The boxes show the stats for the HOP and for the tempo effort that followed.
After I finished the HOP, I "hopped" off T7 to do the usual remove leg and liner, dry down and replace before getting back on for the final 30-min of tempo riding.
An interesting personal observation was as the workout progressed, I seemed to get better. The tempo at the end was taxing but not a killer by any measure. No doubt it won't be like that everytime I do it.
All up, a really good quality session. You should give a go one day.