Any idea what these two skills actually do?

UP

Strength is as you know it, a measure of how strong a person is. In football it's a useful skill that allows players to hold onto balls in duel or either being able to dispose the play holding the ball with strength and combine with good skill on tackle is powerful. It is useful skill in all field positions but more needed by defenders. While off the ball refers to player ability to coordinate themself while not holding balls and is mostly related to forwarder players ability to be at the right position in moment of cross into box to contest for header or in through ball moment where the player have to time his position well to break defenders line while onside. Hope those explanation helps:) 

Landlord, 

Superb! Those explanations are the ones that I would like to have written in kind of a "Guide" of the game. Cc´ed Nicmo (Appart from that really appreciate all the efforts invested in this game). 

@Dompropio1

Thank you for you kind compliments, I appreciate that:) 

Good to know this, thanks.

Following this thread, I reviewed the comments from my last match in order to have a guess on the impact that each skill has on the game.

Please note that I have been playing this game for only one month. It would be highly appreciated if a more experienced player could confirm or clarify my hypothesis. 

Look at the following sequence of comments:

Technique vs Tackling: 13: Aarón Granados(MC) has the ball, Luis Ibañez(MC) is near by. How good is Aaron Granados at keeping the possession. How good Luis Ibañez is at tackling will decide if Granados finally keeps the possession after this “first action” of the attack.

Passing vs Intercept: 13: Aarón Granados(MC) goes for a short pass to Antonio Alcalde(SC), will Iván Teixidó(DC) intercept. A good passing skill will mean that the pass (From Granados) is more likely to progress and the second action of the attack will be translated into the third one. A good Positioning skill from Teixidó will mean that he is more likely to intercept the pass.
 

Off the Ball / Speed vs Strength and Tackle (in case is a potential One on One) 13: Antonio Alcalde(SC) may have a chance to long shoot, Rafael Sierra(DR) tries to stop him. Alcaide will execute the shoot if the Off the Ball / Speed skills are higher than those of the Rafael Sierra (For the defender Strength and Tackle (?) will counter attack the Alcalde Skills.
 

Finishing 13: Antonio Alcalde(SC) goes for the long shot! José Luis Córdoba(GK) to defend. Finishing for Alcaide vs Handling for Cordoba.
 

13: Antonio Alcalde(SC) scores!!! 

As for nat fitness, I didn't understand which parameters it takes into account, fitness after a match and injuries?

You can do fitness before or after match depends what you want. Before match is handy if condition is under 99%

I mean the parameter of natural fitness players

How higher NF, how higher condition stay after match

also chance lower for injury and negative random

I got it right then, thank you