Palpitar means 'to palpitate' or 'to throb,' specifically describing a rapid, strong heartbeat. It's the perfect word to unlock the meaning of this classic song.
Selena sings "mi corazón... me empieza a palpitar" (my heart... starts to palpitate) when she sees her crush. This is the feeling that creates the iconic 'Bidi Bidi Bom Bom' sound, which imitates the sound of her heart beating wildly with excitement!
Have you ever had a crush so intense that your body completely betrays you? That is exactly what Selena is dealing with here. The moment her crush walks by (Cada vez que lo veo pasar [Every time I see him walk by]), her logical brain switches off entirely. She literally sings that her heart gets excited and stops reasoning (se emociona, ya no razona [it gets excited, it no longer reasons]). It is ridiculous, sweet, and totally out of her hands.
The physical reactions she describes are hilariously dramatic. Just hearing this person talk makes her knees shake (Me tiemblan hasta las piernas [Even my legs shake]), and her chest starts pounding out of control. The title phrase, Bidi Bidi Bom Bom, is her playful way of mimicking a racing heartbeat (Y me empieza a cantar / Me canta así [And it starts to sing to me / It sings to me like this]). It turns an overwhelming, nervous feeling into pure, bubbly rhythm.