Sometimes in a stranded knitting project there are stretches of stitches in one colour of yarn that span a length of more than 3cm (1.5″). In such areas it is advisable to trap the other yarn (the ‘floating’ yarn) into the main fabric. This is to avoid the tension issues that can result from over-long floats. It also avoids creating loops of yarn that are so big fingers and toes will catch in them.
If you hold both yarns continuously for stranded colourwork (and I strongly recommend that you do so!) then it is possible to trap floats without letting go of either yarn, whether you hold both yarns continental style (in the left hand), or whether you hold one yarn in each hand.
Trapping floats with both yarns held continental style: take your needle point behind the floating yarn to pick up the yarn for the stitch. For a demonstration, see the video below.
Trapping floats with one yarn held in each hand: see my illustrated PDF tutorial, and/or the video below.
Trapping floats when holding one yarn at a time (aka the sado-masochist method*): cross the yarns clockwise before making the stitch, then anti-clockwise after making the stitch.
When trapping floats in consecutive rounds of knitting it is best to stagger the traps. Calculate the placement of your traps so that there is at least one stitch column between trapped floats in consecutive rounds. This minimises contrast colour yarn ‘peeps’ between stitch columns. Where yarn traps are stacked in one stitch column or neighbouring stitch columns the yarn peeps are emphasised.
*Seriously, stranded knitting will be much, much faster, and less angsty (the yarns won’t twist and tangle together), if you learn to hold both yarns at once as described above. See my two-handed stranding tutorial for the method I use.