I can give it to you in SFB terms, where it's in relation to a six sided dice. I believe SFC works from the same basic "chance of success" but converts it to percentages.
We'll use a Connie, for example. It has a breakdown rating of 5 with 1xHET bonus.
When you perform an HET you roll to see if it was successful, or if the stresses on the hull/engines etc. caused a systems failure. You must roll a number lower than the BD rating. In this case, 1 through 4. A 5 or a 6 would cause a breakdown. The HET bonus subtracts 2 from your roll, but is then "used up" and isn't applied to subsequent rolls. So the first time you perform an HET in a Connie it will always be successful. Even if you rolled a 6. Subtracting 2 from the roll, for the HET bonus, would give you a 4. Since 4 is lower than the BD rating of 5, you succeed. Generally, the larger the ship, the lower the BD rating. Generally, FFs will have a BD rating of 6 while DNs have a BD rating of 4 or less. If a ship has a BD rating of 6 then, without the HET bonus applied, it would only breakdown on a roll of 6. If a ship has a BD rating of 4, or lower, then even with the +2 HET bonus applied the maneuver might not be successful. If you rolled a 6 and subtracted 2 for the HET bonus the number, in this case 4, is not less than the BD rating, so you would "breakdown".
Some ships, pirates, nimble ships, PFs, for example, have 2x Het bonuses. These bonuses are applied one at a time. They can not be used to subtract 4 from a single roll in SFB. If a nimble ship had a BD rating of 4 you would subtract 2 from the roll on it's first 2 HET attempts, still making the HET not 100% assured for any single attempt.
SFC does it a bit differently. It converts the BD rating to percentages. A Connie has a 66.67% chance for success, plus 33.33% for the HET bonus, gives you a 100% chance for success on the first attempt. This works fine most of the time. A nimble ship though with a BD rating of 4 or 3 throws a monkey wrench into using the percentages formula when compared to how it works in SFB. If a ship with 2x BD bonuses in SFC has a BD rating of 4 then it will be successful 100% of the time on the first attempt because SFC does add the 2 bonuses together. 50% chance for success (rating of 4) plus 66.67% for the 2 bonuses, gives you 116.67% chance for success on your first attempt. There are only a few ships that this applies to, namely pirate freighters and maybe some others, I'm not certain off the top of my head, so it's not a major malfunction.
Hope that helps