I've experienced this 3 times now in about a year. I have an ACS short shifter. I would shift into first and then gas the car, not that hard but harder than 'easy' driving. The car would jerk like the brake is being applied , unapplied , applied again , etc.... this make for a jerking head movement that isn't too pleasing. I don't hear any grinding so I don't think it's the gears, the shifter (i think) would be engaged and I don't hear anything unusal. Clutch feels fine... If I got this 'problem', I'd just clutch and shift into second and it'll be fine.... I do remember the first and this third time that I was shifting while turning, whether that makes a difference or not, doesn't make sense.