harry potter - How could Barty Crouch Jr. perform such powerful magic?

Barty Crouch Jr. was about 19 when he was thrown into Azkaban. According to this answer, he spent 11 years and 10 months under the control of his father's Imperius Curse--over a third of his life. Toward the end of that time, Barty began to break free of the Curse to some extent, but he had no access to a wand until he stole Harry's wand during the Quidditch World Cup. How, then, could he be an effective teacher and perform the magic required to Confund the Triwizard Cup and later turn it into a Portkey? In Barty's own words,

It would have taken an exceptionally strong Confundus Charm to bamboozle that goblet into forgetting that only three schools compete in the tournament.

Yet Barty spent a longer time unable to use magic than he did studying at Hogwarts. Most people--Muggles, anyway--would see an ability they neglected "grow rusty" after a while. But Barty is able to perform complicated magic as soon as he regains his freedom. How could someone who was barely an adult at the time of his arrest, and who spent the next eleven years Imperiused to his father's will, have such an exceptional knowledge of advanced magic, as well as the ability to use it?

Answer

How did he do the things he did?

Barty Jr. was a highly accomplished wizard.

His father states that he (Crouch Jr) earned passing grades in every subject taught at Hogwarts, something only accomplished by a very few high status wizards;

‘Yes, my son has recently gained twelve O.W.Ls, most satisfactory, yes, thank you, yes, very proud indeed. Now, if you could bring me that memo from the Andorran Minister for Magic, I think I will have time to draft a response …’

Most of the spells he uses aren't that special

With the exception of the confundus charm, most of his spells are within the range of what he would have learned at school or as a junior death-eater under Voldemort's tutelage; Polyjuice potion brewing, curses, the Dark Mark, etc.

He had help from Big V.

At every stage, Barty Jr. is receiving support from Voldemort. It was presumably his idea to confund the cup and he would certainly have explained (in detail) how to accomplish it.

His skills weren't rusty

By the point that Barty manages to steal Harry's wand, he's already been working to overcome his father's Imperius curse for some time. It seems likely that this would keep him intellectually stimulated.

Prison doesn't (seem to) dull the mind.

Note that other long-term prisoners that we see, notably Bellatrix Lestrange, are capable of impressive feats of magic as soon as they gain their freedom. Long spells (pun intended) of incarceration seem to be no impediment to using magic effectively.