What you want as a simple short routine is one of those elastic things..
Its pretty much a wider laccy band..Not all that thick..But you put it from behind your back handing each end in your hands with it wraped round your clenched fist and shoot strait shots..
It makes it harder to throw and because of the elasticity it brings your hands right back..
Do it often enough as a routine and it'll help your speed of your punches and keep you into the routine of bringing your hands right back to your chin..
Sometimes speed is power to.. But jab is more about technique
Its all practice..keep your arms loose as you shoot a jab out fast and tense your arm and snap for the last few inches before you land the jab... lol
Hope this helps