Here’s how it is explained in the NCFM - Capital Markets Module 
The index-based market-wide circuit breaker system applies at 3 stages of the index movement, either way viz. at 10%,15% and 20%. These circuit breakers when triggered bring about a coordinated trading halt in all equity and equity derivative markets nationwide. The market-wide
circuit breakers are triggered by movement of either the BSE Sensex or the S&P CNX
Nifty, whichever is breached earlier.
(a) In case of a 10% movement of either of these indices, there would be a
one-hour market halt if the movement takes place before 1:00 p.m. In
case the movement takes place at or after 1:00 p.m. but before 2:30 p.m.
there would be trading halt for ½ hour. In case movement takes place at
or after 2:30 p.m. there will be no trading halt at the 10% level and
market shall continue trading.
(b) In case of a 15% movement of either index, there would be a two-hour
halt if the movement takes place before 1 p.m. If the 15% trigger is
reached on or after 1:00 p.m., but before 2:00 p.m., there shall be a onehour
halt. If the 15% trigger is reached on or after 2:00 p.m. the trading
shall halt for remainder of the day.
© In case of a 20% movement of the index, trading shall be halted for the
remainder of the day.
If the trading halts at 10%, and restarts after 1 hour break, will it be allowed to move up above 10%?
Otherwise, it could never reach 15 or 20%.
Is my understanding right?