Getting more leverage in koshi-waza

I should point out at the start that I'm not a judoka myself but I'm seeking opinions and tips on behalf of my son who's been doing Judo for about 2 years now and has has been doing level 1 competitions for the past year.

At the most recent competition we attended last weekend I noticed one player who won his pool executing multiple ippon and morote seoi-nages with his back in an arched position just before performing the throw. His opponent was a little shorter than him so I suspect the arched back may have made it easier for him to get his hips into position.

I did ask my son to try some uchikomi on me adapting his hip throws with the arched back approach. To me it felt like a stronger throw as he was less reliant on using the sleeve pull to get me moving and the combined action of straightening his back whilst bending forward produced a lot more of an unbalancing force at my hip level.

Strangely though, I've not seen any online demonstrations of koshi-waza using an arched back. All of the demos I've seen so far seem to mirror the classic examples in text books.

So my question is, does arching the back help with hip throws or should it only be used in specific scenarios?