booting to fastboot recovery mode and to custom recovery mode (TWRP), trouble with the buttons on the device (power, vol+, vol-)

I have no trouble booting to fastboot: I simply press power + vol. up simultaneously and that's it.

However, I always have trouble booting to custom recovery, in my case, TWRP. For my model (asus X00PD) I should press power + vol. down and then release, however, I don't know when I have to release or if I have to press power and a bit later the vol. down button, nor do I know if the device only boots to this mode when the usb cable is not plugged in.

advice welcome