TASO members got a good (free) webinar from Arnold Gladson last night, covering the 10-second subtraction (or "runoff," as you please).
I was, however, surprised at how many folks didn't grasp the concept of the types of fouls that could qualify for the 10SS. The foul itself must cause the clock to stop at the moment the foul occurs (and the ball to become dead, if alive before this foul occurs). Some (but not all) examples are: intentional grounding or other illegal passes thrown to conserve time; false start; offside with contact. If the game clock is running, these fouls cause the clock to stop immediately and simultaneously with the foul. Live-ball fouls that occur during the down, but do not cause the ball to become dead and the clock to stop, do not qualify for the 10SS. Some (but not all) examples are: holding; block in the back; block below the waist; pass interference. The ball remains alive after these fouls occur. The fouls, themselves, do not cause the clock to stop immediately and simultaneously with the foul. Yes, the clock will eventually stop to complete the penalty, or for some other reason (such as ball/ball carrier out of bounds, line-to-gain reached, etc.). But, such fouls are not committed for the purpose of getting the clock stopped, so they will not qualify for the 10SS.
Other things that qualify for the 10SS do not necessarily cause the clock to stop when they occur, but do cause the clock to stop when the ball becomes dead, if there is no other reason for the clock to stop. These things are: a helmet coming off (not due to a helmet-related foul), and an injured player. If either of these events occur, the clock is stopped when the ball becomes dead (unless it is the ball carrier - then it becomes dead when the helmet comes off), and it will qualify for the 10SS. However, if there is anything else that causes the clock to stop, such as A reaching the LTG, completion of a penalty, ball or BC out of bounds, incomplete pass, touchdown, touchback, safety, etc., then the offending team gains no game clock advantage by losing a helmet or being injured. In those cases, the loss of a helmet or injury do not qualify for the 10SS.
But, in any event, the game clock must be less than 1 minute in the 2nd or 4th periods and RUNNING for any event to qualify for a 10SS. And the offended team always has the opportunity to decline the 10SS. If associated with a foul, the offended team may accept the distance penalty, but decline the 10SS, if they choose. If they decline the distance penalty, the 10SS is also declined (by rule).
The offending team can avoid the 10SS by taking a team time out (if available). If this event is a helmet off, the team time out would also "buy" the player back into the game. However, that doesn't work for an injured player - an injured player may not participate in the next down - no exceptions (even if over change of period or half-time). They can avoid the 10SS with a team T/O, but the injured player has to remain out for a down.
Helmets off by both teams do not qualify for the 10SS. Injuries by both teams do not qualify for the 10SS.
(By the way, if helmets come off of opposing players, a charged team time out by ONE team does NOT allow the opposing player to return to the game. They must "buy" him back into the game with their own team T/O.)
All of this was discussed in the webinar. These are good things, and everyone should take advantage of these roughly 1-hour webinars. Good stuff.