Ramping Isometrics - which is a specific training method and is not the same as regular isometric training - is an interesting training protocol because it helps build MTB specific fitness in two ways:
1 - Improves the all important Strength-to-Weight Ratio. A major factor strength and power production is the amount of tension you can generate in a muscle and movement pattern. The more tension you can generate the "stronger" you will be.
Since Ramping Isometrics allow you to safely build extremely high levels of tension within MTB specific movement patterns you will improve your trail-strength like no other training method can. Plus, it isn't damaging the muscles like traditional weight training, resulting in less soreness and less muscle growth.
This results in a higher Strength-to-Weight Ratio. The higher this ration is the easier it is to push harder gears and maneuver your bike around on the trail, which is the Holy Grail of MTB strength training.
2 - Improves your High Tension Cardio. One of the things that really separates mountain biking from other cycling sports is the extreme amounts of tension we have to generate and sustain on the trail. I refer to these as High Tension Efforts and they are fueled completely differently by your cardio system than lower tension efforts are.
This is one of the reasons that High Intensity Interval Training has become so popular for mountain bikers - interval training works on this High Tension Cardio side of things and help your cardio system adapt to the specific demands of this type of cardio training. And since Ramping Isometrics have your muscles under tension for 90 seconds they give you a chance to work on your High Tension Cardio in a unique way.
The extreme amounts of constant tension force the body to adapt in ways that not even interval training can. You'll find that your hardest efforts feel easier and that you can recover faster, both of which will improve your endurance and speed on the trail.