If he's talking about it stalling down dirt hills and it has an IRK ... then it's a result of the IRK lacking the flywheel effect to keep the engine turning over smoothly at low revs ... so if he's in too tall a gear or touches the brakes or backs the throttle right off ... the engine stalls and the rear wheel locks ... He has to learn to pull the clutch in when he comes across those and other situations ... and keep the revs up and gently feed the clutch out as he accelerates from low speeds ...
T&G is a smart tech guy and he talks about annoying stalling and carrying on when he had an IRK on his YX150 in a bike used mainly on the road ... so you don't have to be on dirt to cop the same crap ...
outer rotor - Planet Minis
High end model trains run heavy flywheels to drastically improve their performance and running characteristics with NO other mod ... The flywheels allow them to operate super smoothly at crawl speeds and pull heavier loads due to greatly increased traction , they don't stop dead when the power is cut ... they coast to a stop ... That in turn prevents them from jerking and stalling on dirty sections of track where they cop intermittent electrical pickup on the wheels ... the flywheels keep them going until they hit clean track and get steady power again ...
The same goes for an IRK equipped bike ... it'll be a pig down low if the engine isn't perfectly tuned and set up plus the rider compensates for the lack of flywheel momentum ...
Trials bikes which can ride over just about anything including terrain that people have to CLIMB over ... have heavy flywheels ... without them they'd be hopeless ...
In a nutshell ... don't try to trail ride or ride an IRK equipped bike at low revs ... they are only designed for riding where the revs are kept high at all times and the bike is always kept in an easy to pull gear ...
Incidentally another fact is that Super Stock drag racers fit heavier than stock steel harmonic balancers and flywheels to greatly boost their launch off the line capabilities , traction and acceleration thru the gears for better elapsed times ... When you run big ports and cams in a given sized motor with a wide ratio tranny ... the flywheel momentum helps to stop the engine from dropping "off the cam" on upshifts ... that in turn allows for a higher revving engine without bogging on the shift or on take off ...
2 stroke MX bikes which had IRK's were relatively gutless down low and required 6 speed close ratio trannies plus a high ratio final drive to function adequately ... and they needed a multi plate clutch to cope with the tendency for them to be vibrated and lose grip ... When you cam a 4 stroke and lighten the crank and flywheel you really need to do the same things ... Early XR75's were only 4 speed ... Racers fitted special 5 speed close ratio trannies to compensate for the altered torque characterists of modded engines ... The trannies made one hell of a difference even to a stock bike ...