As long as those limbs have not been damaged, you may launch your disc from that area. You can see if your feet or hands are damaged, the areas from which you can launch get smaller and smaller, making shots more difficult to make. Also, I've employed the "one cheek" rule as established in Crokinole. Once you sit down, there's no shifting your chair or moving. You can move anywhere as long as one cheek of your butt is still on the chair.
When you flick the disc, it will land somewhere (hopefully) on your opponent's body. The opponent then gets a chance to flick a disc to deflect. If the attacking disc is flicked away, no damage is done. If it is flicked back to the attacker's body, that becomes a really awesome counter-attack and automatic damage is done.
What stops a player from just flicking a disc as hard as they can is that if a disc lands outside of the body box (the smaller surrounding box around the body), that disc is lost. Otherwise you're able to put the disc back in your arsenal to attack with again. Having 5 attacks, you want be calculated and conservative.
Players roll 1d20 each before each round. The winner (rerolling ties) chooses whether to attack or defend. Like in football, it seems a better idea to defend.
Come over sometime and try it out! I can put up the graphic file as well if you'd like to try. Drop me a line!