Mune-gatame (胸固), the chest hold.

Osaekomi-waza (hold-downs).
Required for: 1st Kai (white belt with a red stripe, JudoScotland); 2nd Kai (white belt with a yellow stripe, JudoScotland); 3rd Kai (white belt with an orange stripe, JudoScotland); 8th Kai (white belt with a black stripe, JudoScotland); 9th Kai (red belt with a white stripe, JudoScotland); 2nd Mon (red belt, British Judo Association); 6th Mon (yellow belt, British Judo Association); 6th Kyu (red belt, JudoScotland); 5th Kyu (yellow belt, JudoScotland).

About
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Chest on chest from the side, arms hugging around uke's trunk and far arm: the simplest-feeling of the side holds, kept by weight and hip movement rather than any particular grip.

Step by step (standard right-handed form)
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1. Your partner lies on their back; come to their right side at right angles, chest facing down onto their chest.
2. Wrap your arms around their trunk and far shoulder, hugging them to you, elbows drawn in tight.
3. Keep your hips low to the mat, legs sprawled wide behind you, or tuck your knees against their side.
4. Press your chest down through theirs and keep your head low on the far side.
5. When they bridge, let your hips float and re-settle; when they turn, slide around them keeping chest contact unbroken.

Key points
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1. Chest glued to chest, at right angles from their side
2. Hug around the trunk and far shoulder, elbows tight
3. Hips low or knees tucked; follow every bridge with your weight

Receiving it
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A broad hugging weight lands across your chest with nothing obvious to attack: no collar grip, no head wrap, just heaviness. Bridges lift them but do not move them unless you turn as you bridge; work an elbow inside and turn towards them. Breathe steadily and tap clearly if you are held and need to stop.

VI judo notes
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The simplest of the side holds to keep without sight, because there are no grips to find: your arms wrap the trunk and your chest does the work. Everything they try reads through the chest, a bridge as a lift and a turn as a roll away from you, and the answer to both is to float and settle. Slide around them with the chest contact unbroken rather than releasing and re-taking.

From Waza Book: https://wazabook.com/techniques/mune-gatame/