During the bike your hips will have been compressed, so when you start your run your lower back tightens up and you’re unable to use your normal gait. Firstly, a professional bike fit is a must. Secondly, doing some simple core fitness exercises 2-3 times per week will help build up lower back and leg strength.
Also, in the 12 weeks leading up to a race, regularly practise (three times a week) transitioning from cycling to running. Concentrate on a natural relaxed rhythm rather than flat-out speed or effort. Keep the session short, just 5-15mins, until you begin to adapt a comfortable running form.
When racing, ease off a little towards the end of the bike leg. You should be removing tension, by reducing your cadence, and preparing for the run well before the bike leg ends. Also try to remember to change hand positions during the bike leg to avoid sitting in a fixed position for too long, and stand up for 15secs every 2mins for the final 8mins and spin 5-10rpm faster while sitting more forward on the saddle.
Jelly legs arise from low energy levels, so make sure you’re taking in enough calories during the third quarter of the bike – not too close to T2 but with enough time so it takes effect in time for the run.