Because it is a rolling monthly offer you can cancel anytime you wish and only pay for the months you want.
When cancelling make sure you follow all the steps on screen approx 4 to 5 steps where it will then show cancelled next to the Month pass on your online account.
You can then keep watching up to the next due payment date.
When you cancelled and gone past the entitled date you can watch up to, you won't be able to restart the sports membership at the previous offered discounted price.
The £20 per month deal is a pretty good price, where the best deal I could squeeze out of NOW was £25 per month.
Just for some further clarity with my first paragraph on my first reply.
That if you only want the sports offer for one month, then fully cancel during the first month (a couple of days before your next due payment), should you want for example the offer for 3 months, then cancel on the third month a couple of days before the next due payment date.
Should you want to keep the offer for the full six months duration, then again cancel again a couple of days beforehand in the final six months to prevent the Sports Membership price auto renewing each month thereafter (until you cancel) back to the normal monthly sports Membership subscription price of £33.99 per month when the deal offer finishes.
Hope that makes some sort of sense.