A key important difference between speed point format and regular pull format is the ability to sub on the fly. This rule is implemented to make things go faster during speed point.
While the play develops, players are always running around trying to get open. If one gets tired, they have the option to sub off onto the sideline while someone else on the team can take their place.
General rule of thumb is to only sub when you are on OFFENSE.
If a substitution is made when on defense, this gives the offense a great opportunity to throw it to the poached player.
If defender 3 subs with defender 19, this gives cutter 3 a great advantage and H2 will be able to throw to 3 easily. 3 could easily pass it up field without any pressure while defender 19 has to catch up.
When substituting, the people waiting to sub need to line up in a orderly fashion. The next person to sub onto the field will be the one closest to the center line of the field (the center line will be marked).
As noticed, H3 subs off with cutter 21. Cutter 21 on the team is closest to the center so they're the next one ready to sub onto the field.
When subbing on the fly, make sure to TAG HANDS with each other and the tag can only be within 10 feet of the sideline. You can't run to the middle of the field and tag hands. Doing this will put too many men on the field and will lead to a turn over (if the other team knows the rules :P ).
As an added benefit, there can be two lines, women separated from men to make the subbing on the fly more fluid and effective.