This would produce the most gains , but if you predict wrong you could be stuck waiting for the price to either come back up or down to get back in. I did this the other day. I sold and bought back in at what I had predicted was going to be the next upswing. However some last second sellers swooped in and dropped the price below my limit. Then it just continued to fall from there. Now I sit and wait for the price to come back up to my purchase price so I can get back at it. I hate waiting. I get antsy. I should have left some money in reserve so I could have at least bought in at the bottom. I got greedy. Now I wait. 
I never go all in when trading. Some of my biggest trades are 5% of my portfolio position size. Going all in is emotionally straining.
Well I guess not necessarily. The amounts you have supplied yourself might be misleading to what he actually meant. But I agree with his strategy to never go all in because you can never be too certain about what is going to happen.
I have a friend who tried trading and he won around 10 Bitcoins just because of a single try. Eventually, though, he got greedy. He did not cash out even when Bitcoin's price is already amounting to $19,750. Look where his greed took him -- of course, he is down to 3 Bitcoins because he lost in trading. What is more is that the price of Bitcoin has declined around the last week of 2017 up until present. Currently he is holding, just like someone in this thread. The lesson to learn here is to never put all your eggs in one basket.