Relative strength is commonly termed strength per pound of bodyweight, or strength without size, and is a highly sought after quality for athletes who need strength without much in the way of extra weight.
As I explained in "The Myth of Relative Strength" article a few years back, much of what is talked about regarding relatively strength is more myth than reality. Practically all strength training methods promote some degree of hypertrophy stimulation and muscle gain/loss in conjunction with any sort of resistance training is largely about nutritional intake. In other words, you could eat like a bird and train like Ronnie Coleman and not gain any size or eat like a horse and do bodyweight training and gain a decent amount of muscle. There are also limits to true holistic transferable strength that can be gained without adding muscle mass.
Having said that, if one were to create a good relative strength protocol you want a good combination of these 2 things:
1. High degree of muscular activation under tension
2. Lack of muscle breakdown
You want to both engage the muscles significantly for decent time under tension while simultaneously limiting the breakdown that occurs under that tension. A high degree of muscular activation can occur with plyo training, power training, or other speed-strength and strength-speed methods, but duration of tension is also important, this is why lifting heavier weights gives better strength gains than lifting light weights rapidly, the muscles are under tension for longer periods of time and THAT extra duration of tension largely influences strength gain.
Looking at those 2 things in my opinion the best relative strength training method is isometric training. Isometric training allows a high degree of muscular activation under prolonged tension but without much in the way of intramuscular movement, it's largely the movement (and thus tearing) of the muscle machinery under tension that causes muscle breakdown, and thus hypertrophy stimulation (and also soreness) all of which we want to avoid.
There is one other benefit to the hard training athlete:
For someone engaged in tons of specific work who needs strength training to really be just a supplemental method, that lack of movement and tissue breakdown makes the strength training less of an annoying stressor to the body and that both allows quicker recovery and spares muscular energy for use in more specific training methods.
Think of the mixed martial artist putting in many hours per week into his martial arts training. You have hours of bag, grappling, and sparring work each week. Hours of conditioning work. Highly fatigued muscles. Where do you fit in strength training and how do you do so as efficiently as possible (eg. keeping the muscles as fresh as possible without creating excessive fatigue)?
I discovered this partly out of necessity during my time spent training as a boxer. I am not an athlete who recovers all that well from strenous training in the first place. I was engaged in about 10 hours of boxing training per week and my upper body muscles were usually like jello. I found that doing anything resembling normalcy in the weight room left me too fatigued. I considered eliminating all strength work but I also lose strength very easily. So the solution I came up with (which worked wonderfully) was isometric training. I would load myself up with weight on the pullup bar and hold the midpoint of a pullups. I'd do dips and bench presses where I'd lower the bar down to the midpoint and hold for 10-20 seconds. I'd do single arm bicep curls where I'd just lower the bar to the midpoint of the curl and hold. I'd do some bodyweight gymnastics variations such as the planche and front lever. Bodyweight isometric training can build a decent degree of strength and do so very quickly.
The result was I was able to maintain and even gain a little strength while keeping my muscles as fresh as possible for boxing.
Keep these points in mind:
1. There are 2 types of Iso, yielding and overcoming. In a yielding iso you "lower and hold" a weight in a given range for a given time interval. In an overcoming isometric you press against an immoveable object. Both are effective but I prefer yielding isos as the force is more easily monitored and they are a bit easier on the joints.
2. When possible try to do your isos at the weakest part of a movement, which is usually the "stretch range" of a movement. Research has shown strength gained in the "stretch range" of a movement carries over to the entire movement. In the bench press that would about 1/3 of the way up and in the pullup that would be about 2/3 of the way up.
3. Use holds of 20 seconds or less. Longer holds up to 1 minute do have utility but you'll build most useable strength using hold times anywhere from 5 seconds up to 20 seconds. The ideal length of time varies a bit with the particular movement. For example, 15 seconds on a bench press is easy enough but 15 seconds on a squat is a totally different animal. If in doubt stick to around 10-20 seconds per set.
Here is a sample 2 day routine - each session would be done once per week for 2-4 sets per exercise:
Yielding iso bench
yielding iso pullup
yielding iso dip (at midpoint)
Yielding iso preacher curl (about 1/3 up from the bottom)
yielding iso deadlift (lower and hold just below knees)
yielding iso barbell rollout
yielding iso military press
yielding iso one arm db row
yielding iso planche hold
yielding iso front squat
yielding iso glute ham or leg curl
yielding iso kneeling pallof press
Give some of these methods a shot. You might be pleasantly surprised at the immediate gains you make. Isometrics really emphasize the "duration" of tension and that is a novel stimulus for most people.