I really don't believe you can react fast enough to defend against a reverse punch or a jab or often even quite powerful attacks in a specific manner. Most good fighters/boxers/karatekas will try to prevent over-extension and always pull their arm back as quick as possible, often when the next strike in their combination of hits is about to occur.
You cannot defend against any specific attack because you have to initiate your entering movement as the attack starts to be made (as is constantly practised with boken work). My technique involves raising both arms to a jodan (high) position whilst moving in (and off centre line). This has many advantages ->
1. if it is a low strike or kick you are out of the way of it and your arms can quickly lay across their arms/legs for a technique (even if they withdraw quickly you should be close enough to their body to do this)
2. moving off centre line and into close body contact should avoid the impact of any strike; in fact the strike really shouldn't be completed because you need to make contact as soon as possible.
3. If they are strong you have both your arms up their feeling their resistance and you can lead this to whatever technique is appropriate.
4. your strong entering movement should disconcert them and break their balance anyway. - as was mentioned by Ueshiba in a Doka of the day; even though there are whirling swords/fists around you, you MUST enter. Backing off from a technique just puts you in a great position for their next strike.
make contact as soon as possible (if you know they are going to attack, don't wait for the attack to occur).
keep your guard up, but use it also to make contact with the person (esp. arms etc) and do what is most appropriate to that position.