This exercise is straight forward and requires NO equipment so you can do it any time anywhere (I imagine you may get a few weird looks in certain locations but don’t let that stop you!) If you do it with good technique and with attention to detail (ie. keep the shoulders rooted down and legs straight) then it is really intense through the core.
Progression (if you need MORE): Add weight to the legs or push less through the arms. SLOW DOWN, put the brakes on!!!
Modification (a slightly easier version): Keep legs bent at 90 degrees making sure that your knees are over your hips or reduce the range of movement so that you don’t go down as close to the floor.
Do these till your form fails you. Rest for a bit and go again. No laughing or coughing the next day (that could hurt :P)
Keep training, Keep playing