So Reverse and Cross are usually just different names for the same thing - so I will ignore the second move in the combination.
Jab vs Back fist (outwards)
- Rotation - a jab will start with the thumb up (or to the outside for full rotation) and rotate so that the thumb is to the inside as with a normal punch - backfist will start with the thumb to the side (or even down) and rotate to thumb up.
- Start position - the jab will be fired straight from a guard (usually in front of the face and between the two fighters) - backfist will be fired from further back (the hand at least) with the elbow used to guard instead (in the case of a weak jab-like backfist)
- Movement of technique - jab moves in a straight line to the target - backfist moves in an arc away from the attackers centre line.
- Tool (more obvious when not in gloves) - the jab is with the forefist - the backfist is with the backfist (same knuckles but the top rather than the front).
- Body Motion - if you are from the school of hip twist for power - the jab will be powered by rotating the front of your body away from the opponent - the backfist will be powered by rotating the front of your body toward the opponent.
So the jab and backfist are very different - which changes the overall combination somewhat - jab/cross comes slightly from the jabbing hand side - then the cross hand side (so that familiar head bob defense is often used). The backfist/reverse both travel from the cross hand side - so general defense is to commit to movement to one side.
Or looking at it from a bag work perspective - jab and cross hit the same part of the bag directly infront of you (at the point you fire the techniques - you may side step etc first). The backfist will hit the opposite side of the bag to the starting hand and the reverse will be between that and the front (near where the cross would have hit in jab/cross - but the bag will have been pushed to the side by the backfist so it will feel more to the side)
What about a downwards backfist?
Tool is the same as the outwards backfist - the rotation will be the same way too - but the fist rotates so the backfist is pointing down instead of out. The Direction is an arc downwards instead of an arc down and the start position will be close to the body with the elbow raised.
I will omit much of the talk on this technique because raising the elbows is a good way of inviting body shots.