Learn Portuguese (Portugal): Se fores um bom esquiador e que faças isto rápido, ganhas. - If you are a good skier and you do this fast, you win.

It acts as a stand-in for the word se (if) to avoid repeating it. Instead of saying se fores... e se fizeres... (if you are... and if you do...), Portuguese uses this replacement que (known as the vicarious que) to connect the second condition.

The first verb fores is in the future subjunctive, so why is the second verb faças in the present subjunctive?

This is a specific quirk of replacing se with que. When the vicarious que replaces a conditional se, the verb that follows it shifts from the future subjunctive (fizeres) to the present subjunctive (faças).

Could I use rapidamente instead of rápido here?

Yes, you could say faças isto rapidamente, but using the adjective rápido as an adverb is very common and natural in conversational Portuguese. It means the exact same thing but sounds a bit punchier in everyday speech.

Why is the final verb ganhas in the present tense instead of the future tense?

While you could use the future tense (ganharás), using the present indicative (ganhas) for the result of a likely condition is extremely common. It makes the consequence feel immediate and certain, much like saying 'you win' instead of 'you will win' in English.