I recently heard a great quote by Pavel Tsatsouline along the lines of:

'Master muscle tension and you master strength and power, master muscle relaxation and you master flexibility, speed and endurance'

But what about endurance training? If you were to do say 20 chins or 50 H.Squats what degrees of tension and/or relaxation would you be wanting to generate?

And what about explosivity? Say you are throwing a medicine ball, how would you get the balance of strength and speed?

All you have to do is train what you want your body to do. Look at olympic gymnists... They are strong, flexible, and enduranced based... That's why there are different sets, reps, tempo, and rest periods for endurance, strength, power, etc...

For endurance, you do something like 2 sets of 15-20 reps at a 4-2-2 tempo and only about 50-60% of your 1 RM. Power, is 3-5 sets of 3-5 reps at a 1-0-1 tempo at 90%+ of your 1 RM.

Your medicine ball question all you do is start with a nice light medicine ball and throw it with a nice easy speed, but it HAS to be controlled, then slowly speed up as long as you can MAINTAIN that control... Once that gets easy, all you have to do is jump the weight up, and go a nice speed and controlled, and then speed up...

When I do a set of pure strength work I use Pavels hyperirradiation technique (tensing the body basically) to great effect. If I want to move superquick I find that relaxing the whole body is excellent. So when doing strength-endurance work should I be more tensed or relaxed?

Just tense the muscles that you are working... so like with a bench press, at the top of the press you'll want to 'squeeze' your chest muscles and maybe even hold for a 2-3 count...