Most has already been said, but I'll just add a bit more based on watching many kills in my garden:
Firstly; Sparrowhawks don't have a "Kill shot", if the prey dies from the initial strike, it's just luck. Sparrow-sized birds are usually the most fortunate, anything bigger and it's nearly always a very slow death.
A few Sparrowhawks develop a habit of killing quite quickly, in fact it can be instantaneous, but it's a bit "nasty", so I won't explain.|=)| The Sparrowhawk will insert it's Talons where it can (neck and chest are favoured most often) and retract, reinsert, wiggle about, etc. IMHO, it's not trying to kill the prey, it's just trying to stop it from struggling, by weakening it. Basically, as soon as the prey has been restrained, the Sparrowhawk begins plucking and eating. Just to be clear "plucking and eating" means pluck - eat - pluck - eat, etc; I've never seen a bird plucked, and then eaten. (BTW, if you can't find examples elsewhere, my gallery has loads of pictures of Sparrowhawks restraining the prey in different ways.) Often the prey appears to go unconscious, only to wake and fight again even though it's been half-eaten.
None of this is "nice" of course, but what else can Sparrowhawks do? They either eat or die, and Nature hasn't given them the ability to kill their prey instantly.
I've rushed this a bit, as I go offline soon, but I've not missed too much out.