Learn Portuguese (Portugal): Puxa o peso devagar!

It actually means "pull"! This is one of the most famous false friends in Portuguese. If you want to tell someone to push, you have to use the verb empurrar. Many English speakers get confused at doors in Portugal because they push when the sign says Puxe (the formal command to pull).

Since this is an informal command (for "tu"), why isn't there an "s" at the end like puxas?

To make an affirmative command for tu (informal "you"), you just take the normal present tense tu form (tu puxas) and drop the final "s". So, puxas becomes puxa. It ends up looking exactly like the "ele/ela" (he/she) present tense form.

Could I use lentamente instead of devagar to say "slowly"?

Yes, you can! Lentamente and devagar both mean "slowly" and are often interchangeable. However, devagar is very common in everyday spoken Portuguese, especially in short commands like this one or when telling someone to slow down (Mais devagar!).

How is the "x" pronounced in puxa?