I feel that the powertwister and other spring devices are very underrated for core strength development and especially wrist and lower arm strength. The chest is not not really challenged that much IMO, if you bench heavy. Arm wrestlers love them .
They require valsalva maneouvre style breathing at an intermediate to advanced level.
This is what a spring looks like with a 10 rep max.
Like everything they are not without their risks . I sprained my wrist trying to get to the next level.
Try these devices at home and travelling. Not marketing or recommending any particular brand.
This is quite hard to close. This is the one thatās given me the 10 week wrist sprain when I tried without assistance. I got close lol
They are great for wrist strength. The new gold series weaken very little . Quality German style. There are a few different brands.
I will stop posting videos now lol. My Vimeo quota is used up.
Spring can tolerate small weights easily btw. Resistance is about 100kg despite the 90kg tag. The assigned weights are fairly arbitrary.
This looks like an excellent way to knock your teeth out 
They can be dangerous but so can lifting weights.
I found good carryover in core strength to the squats.
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I am trying to get to the next level. Unfortunately I sprained my wrist with this attempt.
The people who can close it tend to be good grip athletes who fairly large for the most part.
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