The "Logsplitter" (Biceps) Guard Opening
Opening the closed guard is probably one of the toughest areas of proficiency in all of jiu jitsu. It's incredibly difficult to be inside your opponent's guard -- not a great place to be -- but to have the onus on you to get their legs to open up so that you can advance your position. As a result, a closed guard player must simply remain patient, and wait for the right opportunity to attack. The guard opener, on the other hand, must necessarily risk a great deal in order to open up the guard. However, the "logsplitter" guard opening, involving biceps control, creates one sneaky way around this problem.
The logsplitter typically begins when your opponent begins to control your posture (although you can certainly go directly into the position without waiting for any sort of trigger). Stop their eventual grasp of control by planting your hands firmly on their biceps, either with your thumbs pointing up (as shown in the video above), which facilitates your elbows staying in closer to your body, or with your thumbs down, which facilitates easier access to downward, pinning force. Either way, your next move is essentially to create an A-frame house with your body, with your hips at the apex of the letter A. Next, the idea is to weave your lead knee (let's use the right knee as an example here), while curtsying into a combat base position. One option to finish is to lean back even further than the position requires, then switch to a leg drag pass, as shown in the video.
A simple, quick adjustment is to involve the gi with the initial grips. When your partner breaks your posture down, grasp their lapels about midway, allowing you to extend your arms, with your knuckles ending up in their armpits. The idea here is for your fists to pin their arms, facilitated by the gi lapel wrapping around and underneath their armpits, ultimately (in turn) pulling their shoulders down and pinning them to the mat. Needless to say, this can help get even a stubborn closed guard open.
Sometimes, in the course of attempting a more traditional logsplitter opening, you will inevitably encounter a person who isn't interested in opening their legs at all, and they have the physical bandwidth to avoid succumbing to the opening. Here, it pays to attempt to wiggle your hips backward ever more. Sometimes this is just enough, but whenever it isn't, try pushing down on your partner's hips. Be careful here, though, as you are also freeing up their arms, leading to their likely attempts to sweep you (in a frenzied panic, most likely).
One key detail with this position is to keep your elbows in, close to your body. If your elbows flare out, your partner can swim inside your hands and break your posture much more easily. If you are choosing to pin the biceps with the "thumbs up" posture, just be conscious of the opportunity for your partner to creep their arms up closer to your legs, or possibly to take control of one of your sleeves (possibly using a cross grip sequence).
Logsplitter, or Open from the Knees?
The logsplitter guard pass offers a fantastic answer any time your partner seeks to control your posture. Allow them to pull you forward, but use the opportunity to advance your position by taking control of their biceps. From there, you have a solid opportunity to open their guard (and then pass). You may need to sit back down repeatedly during the learning process, and even when your're particularly good at the position, you might need to sit down from time to time, whenever your partner seriously threatens your balance. However, you have a solid plan to get back to opening the guard you can use right away. As always, please let me know if this technique is working for you!