So Harry's broom was jinxed by Professor Quirrell in the Philosophers Stone and he was knocked over by Miss Granger, while Snape was doing a countercurse.
‘No, no, no. I tried to kill you. Your friend Miss Granger accidentally knocked me over as she rushed to set fire to Snape at that Quidditch match. She broke my eye contact with you. Another few seconds and I’d have got you off that broom. I’d have managed it before then if Snape hadn’t been muttering a countercurse, trying to save you.’
Philosopher's Stone - page 209 - Bloomsbury - chapter seventeen, The Man With Two Faces - Harry Potter and Professor Quirrell
However according to the incident, it took something like 30 seconds between knocking over Professor Quirrell and the fire disrupting Snape, and it's only after the later than Harry regains control of his broom.
Hermione has fought her way across to the stand where Snape stood and was now racing along the row behind him; she didn't even stop to say sorry as she knocked Professors Quirrell headfirst into the row in front. Harry Potter and the Philosophers Stone, Chapter Quidditch
At this point the Jinx should be broken but it's not and the counter-curse is still ongoing at this point
It took perhaps thirty second for Snape to realise that he was on fire. A sudden yelp told her she had done her job. Harry Potter and the Philosophers Stone, Chapter Quidditch
And it's only after this that Harry re-gains control of his broom.
Up in the air, Harry was suddenly able to clamber back on to his broom. Harry Potter and the Philosophers Stone, Chapter Quidditch
My question is two fold, why did the jinx not fail once Quirrell lost eye contact ?
And why only after the countercurse was stopped was Harry able to regain control of his broom?
I think Snape was up to something.