Double under pass (stack pass) in BJJ: both arms under the legs, stack the opponent, pass to the side. IBJJF rules on stacking, slams and the neck, safety.
The double under pass (also stack pass) is a pass in which you bring both arms under your partner's legs, stack him and pass to the side. That is how the DOMISPORTS skill catalogue describes it. Where exactly your arms lie, how you use your weight and how far you stack your partner is shown by your coach. It is delicate and depends on the school, and this text cannot replace it.
The honest answer: the double under is a pass with pressure, and pressure is the number one topic for this move. Whoever stacks his partner loads the partner's back and neck. The rulebook sets clear limits for that, and you should know them before the first repetition. How long you need for a safe pass cannot be stated; there is no study on it.
The IBJJF rulebook (version 6.1, 2024) names the stack explicitly. Under Art. 5.6.2 there is an advantage point when the athlete passing the guard stacks the opponent or lets the opponent roll back and then controls his back on all fours, with at least one knee on the ground, without placing hooks [1]. The full guard pass of 3 points you get when you hold side control or north-south for 3 seconds [1].
The prohibitions are just as clear. In the table of illegal techniques, slam, spinal lock without choke and suplex takedowns that put the opponent's head or neck on the ground are listed for all classes [1]. That is the frame within which you learn the double under.
The order follows the catalogue's idea: arms under the legs, stack, pass sideways. The exact arm path and weight shift are shown by your coach. Every step is practised first slowly and with agreement.
You open the closed guard. Milestone: you open it in five of ten attempts against light resistance.
You bring both arms under your partner's legs as your coach shows. Milestone: five repetitions without pressure that your coach confirms as clean.
You stack your partner slowly, only as far as he goes along with it. Milestone: five repetitions in which your partner can tap or stop at any time and you yield immediately.
You move past the legs sideways. Milestone: you pass slowly in three of five attempts against a passive partner.
After passing you hold side control. Under Art. 4.2, 3 seconds count [1]. Milestone: you hold control for 3 seconds before you release it.
In the double under your weight rests on your partner, and he is stacked. The rulebook prohibits slams, spinal locks without choke and suplex takedowns with head or neck on the ground in all classes [1]. An evaluation of tournaments in Hawaii counted 7 of 46 injuries as rib or costochondral injuries [2], and a systematic review names chest and rib region next to knee and shoulder as frequent sites [3]. There are no data on the double under itself. As an inference: never stack your partner quickly, stop at once when he taps or raises a hand, and practise the move only under supervision and with agreement. Whoever is being stacked taps early. This guide explains and motivates; it does not replace supervised training.
Agree on a stop word with your partner before the round. One signal is enough, but both must know it. It also helps to swap roles: whoever has been stacked once understands why pace and dosage matter. That is experiential knowledge, not a study. Also ask your coach where he draws the line between a controlled stack and overstretching in class. The answer says a lot about how the school handles injury risk.
A pass in which you bring both arms under your partner's legs, stack him and pass to the side. That is how the DOMISPORTS skill catalogue describes it.
The rulebook gives an advantage point when you stack the opponent or let him roll away and then control his back on all fours [1]. The full pass of 3 points you get with side control or north-south over 3 seconds [1].
There are no data on the move. The rulebook prohibits slams and spinal locks without choke in all classes [1]. So never stack quickly and stop at every sign from your partner.
The catalogue names opening the closed guard as a prerequisite so that you can start the double under at all.
Smash Pass in BJJ: Flatten the Half Guard and Free the Leg