If you are doing leg yield, you WANT forward movement. I think Sidepass is predominantly sideways movement. Leg yield should be about 50/50 sideways and forward movement.
I was taught to start by going forward, then (leg yeilding to the left), put a tiny amount of flexion in the jaw toward the right (away from the diretrion of the sideways movement), and start asking the horse to step across and under with his outside hind (right) , and doing the asking in a pulsing manneer, such that the asking coincides with his lifting that leg off the ground and placing it.
If you feel the barrel of the horse swing to the left each time his right hind steps forward, you will be asking for that sideways step right as the barrel starts to swing leftward. At the same time I give a little tickly or tiny squeeze of the rein on the same side (right). With the horse's next step, you change your focus and think "support" him on his outside (outside of his bend) left rein and kind of hold it firmly to gently stifle sideways movement and encourage forward movement instead.
So, in effect, you are asking for one step sideways, one steop forward, one step sideways, one step forward. A really good ride will meld that together, but a more low level rider such as myself needs to keep the two stages seperate in my mind to remember to do each one and not get stuck simply focussing on sideways only. If you lose your forward imputus, things fall apart. So, if that does happen, abandon the leg yield, ride forward and reestablish "forward!"