When using traditional jump leads, the negative connection to the dead car is the last connection you make - this completes the circuit. The donor battery is 'always on' - there is no switch. So when you connect the lead, you complete the circuit. At this point, once the circuit completes, you can get a spark from the negative lead.
The problem is that batteries can vent explosive gasses, so the spark right on top of the battery is a bad idea!
Most jump packs have a switch, and as long as you attach the leads with the pack switched off, it wont spark. Jump packs also usually have some protection built in to make sure you dont attach the leads the wrong way around.
It's good practice to use unpainted metal for the negative if you can reach it, but with the short leads (especially on lithium iron battery packs) its not always possible. Check your jump pack - it will give you the safe instructions to use it.