Was it ever explained how Goku could use the Instant Transmission while charging a Kamehameha? He didn't have to touch his forehead with his fingers and he never did this again. :
Son Goku Instant Transmission Ver Dragon Ball Z Prize Figure
Instant Transmission (@InstantTransmi1) / Twitter
Goku Instant Transmission (Shunkan Ido) Airborne by dabbido on DeviantArt
Dragon Ball Finally Let Vegeta Learn Instant Transmission