In typical combat, the order of actions is determined (in my campaign) by an Agility roll (representing how fast your reflexes are). The higher the roll, the earlier you act in a round. We'll use the example of a gun duel to show how Haste Actions play into this. Han is fighting Gallandro. Han rolls lower than Gallandro, so when they act, Gallandro would go first. Han, however, decides to declare a 'Haste' on his first action (a shot). This would subtract 1D from his chances to succeed with that shot, in addition to any multiple action or wound penalties he already had. He would get to fire before Gallandro. Also, since Han's shot is hasted, Gallandro would have to declare a 'Haste' Dodge in order to have any chance of avoiding the shot (i.e. he would have to subtract 1D from his dodge).
That seems relatively cut and dry, until you factor in multiple hastes—whereby (in the above example) Gallandro could declare a haste action to negate Han's and still shoot before Han's haste. The escalation goes on from there, limited only (at least to my knowledge) by the level of the skill that you're hasting. In my own game, we had to eventually limit this kind of thing, because when highly skilled characters (8D+) start spending force points and using hastes, it wasn't out of the question to have a 10-12D haste action.
This may seen excessive, but it was useful in doing what my players termed 'suppressive fire'. Essentially, one or more characters would do multiple haste actions versus an opponent. That opponent would have to do a multiple haste dodge to avoid that shot—thus lowering his dodge roll for the round. This would mean that anyone else shooting at the target—without hasting—would have an easier time to hit. In a way, this made sense to me. Suppressive fire is a real-world tactic and it works on those same principles. Someone lays down covering fire, makes the target react hastily so that someone else can draw a bead on them.
But despite liking the theory, in play, the tactic got to be a bit unbalanced. I didn't want to throw it out entirely, though, so I wound up coming up with a few restrictions. First of all, you can only declare as many hastes as you have full dice in agility. You have a 3D agility, you can declare up to 3D haste. Secondly, the person who won the initiative in a combat round can ALWAYS choose to match the number of hastes that the other player wants to use (and thus, go before they do anyway). This has eliminated both the ridiculous 12D haste actions and the one-upsmanship in declaring numbers of hastes between combatants. But it hasn't made haste actions useless. A good gunman can still lay down a respectable suppressive fire.
Some of my players grumbled at the implementation of the new rule, but oh well. I think it is a lot more balanced now. So... nyah!