For any given preset, you can specify a transition to get the lights from their current state into the colors of that preset. This can be a simple crossfade or an attention-getting strobe. Transitions are applied to all lights involved in that preset. To access controls for a transition, go into the preset editor and select the button to the left of your color assignments. This will show four different options for this preset's transition:
Crossfade lets you move gradually into the preset, using a fade time you specify. As with any duration slider in Lightbow, you can use the knob to choose anything between zero and five seconds on quarter-second intervals, or tap the value itself (in this case "1 sec") to bring up an editor giving you more control, like more precise increments or durations longer than five seconds.
Fade Through Color lets you choose a single color and duration to fade through on the way to the presets actual colors. For example, glide through a nice midnight blue:
Flashbulb is basically a strobe light, with a duration you specify. Note that this only works on LIFX bulbs since Philips deliberately removed this feature from their bulbs and hubs in late 2015. I have contacted them about this multiple times, and despite general user uproar, they have decided this is a feature they do not want to pursue or maintain. I left this option visible in case Philips users are on old firmware versions, since I know there are some of you that rely on strobing for your light shows or art installations.
Finally, there's "Use Default Transition" which is a crossfade whose duration you can set in the Settings tab. This is handy if you don't want to customize all your individual presets, and just want one handy place to change the fade time for everything.