Holding it in doesn't require the use of a hand (unless you're using motakasha's method from above ... have to try that sometime). The same muscle that enables you to stop peeing is the same muscle that you flex to hold the ejaculation back. You can only hold it back for a couple of seconds, but its worth it.
Its the same muscle that you can flex when peeing to make it come out in high pressure bursts.
Practice makes perfect.
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