The jab grab is an advanced technique in Smash 64 where a player can cancel the first hit of their neutral attack with a grab. It is performed by doing a jab and then quickly grabbing (within the first three frames of the jab).
While the technique can technically be performed by every character, it is only useful with characters who have a frame 2 jab (which includes Mario, Luigi, Pikachu and Ness). This is because these characters can connect their jab on frame 2 and then on the next frame, they can cancel the jab into the grab, while other characters cannot get their jab out in time if they perform the techique. The additional hitlag inflicted also makes the timing for the jab grab much more lenient. When the technique is performed, the jab will combo into the grab, both on hit and on shield, making it a very fast way to combo into a grab, as well as a quick and simple form of shield pressure. It can also be used as a faster alternative to a raw grab as the opponent will have less time to avoid a jab grab, as opposed to a grab on its own (since the jab comes out on frame 2, while the grab comes out on frame 6).
Level 8-9 computer opponents occasionally use this technique as well, more often with Mario and Pikachu.
This technique is highly useful in competitive play, and it is one of the key reasons why certain characters are considered better at pressuring, as not having this technique highly limits shield pressure.